test(e4): extract PLEX_SUB_URL for round-trip coverage

Move `type Sub` + `PLEX_SUB_URL` out of the 538-line PlexBrowse into a small
plexSubUrl.ts so the encode/decode round-trip and its guards can be unit-tested
without dragging the component (and its lazy hls.js player) into the test graph.
Pure code motion — PlexBrowse imports the two symbols; the `SubviewUrl` import
moves with them.

Tests lock: encode of each bookmarkable kind, grid/player → null, encode→decode
identity, a person name containing ':' round-tripping, and the decode guards
(positive-integer playlist id; season needs both ids; non-empty show/info/person
arg; unknown kind/garbage → null).

12 tests; full FE gate green (tsc/eslint/prettier/knip).

frontend/src/components/PlexBrowse.tsx

@@ -21,7 +21,8 @@ import {
 } from "../lib/api";
 import { plexLang } from "../i18n";
 import { useDebounced } from "../lib/useDebounced";
-import { useHistorySubview, type SubviewUrl } from "../lib/history";
+import { useHistorySubview } from "../lib/history";
+import { PLEX_SUB_URL, type Sub } from "./plexSubUrl";
 import { toggleWatchState } from "../lib/plexWatch";
 import SortControl from "./SortControl";
 import { PageToolbar } from "./PageShell";
@@ -36,63 +37,6 @@ import { PlexInfoView, PlexPersonView, PlexSeasonView, PlexShowView } from "./pl
 const PlexPlayer = lazy(() => import("./PlexPlayer"));
 const PlexPlaylistView = lazy(() => import("./PlexPlaylistView"));
 
-type Sub =
-  | { kind: "grid" }
-  | { kind: "show"; id: string }
-  | { kind: "season"; showId: string; seasonId: string }
-  | { kind: "player"; id: string; queue?: string[] }
-  | { kind: "info"; id: string }
-  | { kind: "person"; name: string }
-  | { kind: "playlist"; id: number };
-
-// Mirror the Plex sub-view to ?v=<kind>:<args> so a show / season / movie-info / person / playlist page
-// is bookmarkable. Grid (root) and the ephemeral player are not mirrored (encode → null). The person
-// name is the whole tail after the first ":", so a name containing ":" round-trips.
-const PLEX_SUB_URL: SubviewUrl<Sub> = {
-  key: "v",
-  encode: (v) => {
-    switch (v.kind) {
-      case "show":
-        return `show:${v.id}`;
-      case "season":
-        return `season:${v.showId}:${v.seasonId}`;
-      case "info":
-        return `info:${v.id}`;
-      case "person":
-        return `person:${v.name}`;
-      case "playlist":
-        return `playlist:${v.id}`;
-      default:
-        return null; // grid + the ephemeral player are not URL-mirrored
-    }
-  },
-  decode: (raw) => {
-    const i = raw.indexOf(":");
-    const kind = i === -1 ? raw : raw.slice(0, i);
-    const arg = i === -1 ? "" : raw.slice(i + 1);
-    switch (kind) {
-      case "show":
-        return arg ? { kind: "show", id: arg } : null;
-      case "info":
-        return arg ? { kind: "info", id: arg } : null;
-      case "person":
-        return arg ? { kind: "person", name: arg } : null;
-      case "playlist": {
-        // Positive-integer id only — a fractional/zero/negative ?v=playlist:<arg> would open a
-        // sub-view for a non-existent id (matches the ?pl guard in PlaylistsProvider.readSelectedId).
-        const id = arg ? Number(arg) : NaN;
-        return Number.isInteger(id) && id > 0 ? { kind: "playlist", id } : null;
-      }
-      case "season": {
-        const [showId, seasonId] = arg.split(":");
-        return showId && seasonId ? { kind: "season", showId, seasonId } : null;
-      }
-      default:
-        return null;
-    }
-  },
-};
-
 // The Plex module's content area: browse/search the mirrored library as poster cards, drill from a
 // show into seasons/episodes. Library scope / watch-state / sort live in the left PlexSidebar; the
 // search term comes from the shared top Header search box (q). A show drill-down rides history

frontend/src/components/plexSubUrl.test.ts

@@ -0,0 +1,69 @@
+// PLEX_SUB_URL encode/decode round-trip + guards. Extracted from PlexBrowse for this test (E4
+// deep-review deferred gap). Pure — no DOM, no history.
+import { describe, it, expect } from "vitest";
+import { PLEX_SUB_URL as U, type Sub } from "./plexSubUrl";
+
+describe("PLEX_SUB_URL encode", () => {
+  it("mirrors each bookmarkable sub-view to ?v=<kind>:<args>", () => {
+    expect(U.encode({ kind: "show", id: "s1" })).toBe("show:s1");
+    expect(U.encode({ kind: "season", showId: "s1", seasonId: "se2" })).toBe("season:s1:se2");
+    expect(U.encode({ kind: "info", id: "m9" })).toBe("info:m9");
+    expect(U.encode({ kind: "person", name: "Jane Doe" })).toBe("person:Jane Doe");
+    expect(U.encode({ kind: "playlist", id: 12 })).toBe("playlist:12");
+  });
+
+  it("does NOT mirror the root grid or the ephemeral player (encode → null)", () => {
+    expect(U.encode({ kind: "grid" })).toBeNull();
+    expect(U.encode({ kind: "player", id: "m9" })).toBeNull();
+  });
+});
+
+describe("PLEX_SUB_URL round-trip", () => {
+  const cases: Sub[] = [
+    { kind: "show", id: "s1" },
+    { kind: "season", showId: "s1", seasonId: "se2" },
+    { kind: "info", id: "m9" },
+    { kind: "person", name: "Jane Doe" },
+    { kind: "playlist", id: 12 },
+  ];
+  it.each(cases)("encode→decode is identity for %o", (view) => {
+    const raw = U.encode(view);
+    expect(raw).not.toBeNull();
+    expect(U.decode(raw as string)).toEqual(view);
+  });
+
+  it("a person name containing ':' round-trips (tail after the first colon)", () => {
+    const view: Sub = { kind: "person", name: "Dr: Strange: Origins" };
+    expect(U.decode(U.encode(view) as string)).toEqual(view);
+  });
+});
+
+describe("PLEX_SUB_URL decode guards", () => {
+  it("rejects a playlist id that is not a positive integer", () => {
+    expect(U.decode("playlist:")).toBeNull(); // Number("")===0, not >0
+    expect(U.decode("playlist:0")).toBeNull();
+    expect(U.decode("playlist:-1")).toBeNull();
+    expect(U.decode("playlist:1.5")).toBeNull();
+    expect(U.decode("playlist:abc")).toBeNull();
+    expect(U.decode("playlist:12")).toEqual({ kind: "playlist", id: 12 });
+  });
+
+  it("requires both ids for a season", () => {
+    expect(U.decode("season:s1")).toBeNull();
+    expect(U.decode("season:")).toBeNull();
+    expect(U.decode("season:s1:se2")).toEqual({ kind: "season", showId: "s1", seasonId: "se2" });
+  });
+
+  it("requires a non-empty arg for show / info / person", () => {
+    expect(U.decode("show:")).toBeNull();
+    expect(U.decode("info:")).toBeNull();
+    expect(U.decode("person:")).toBeNull();
+  });
+
+  it("rejects an unknown kind or garbage", () => {
+    expect(U.decode("grid")).toBeNull();
+    expect(U.decode("player:m9")).toBeNull(); // player is not URL-restorable
+    expect(U.decode("bogus")).toBeNull();
+    expect(U.decode("")).toBeNull();
+  });
+});

frontend/src/components/plexSubUrl.ts

@@ -0,0 +1,61 @@
+import type { SubviewUrl } from "../lib/history";
+
+// The Plex module's sub-view union and its address-bar mirror, extracted from PlexBrowse so the
+// encode/decode round-trip (and its guards) can be unit-tested without pulling the whole component
+// (and its lazy hls.js player) into the test graph.
+export type Sub =
+  | { kind: "grid" }
+  | { kind: "show"; id: string }
+  | { kind: "season"; showId: string; seasonId: string }
+  | { kind: "player"; id: string; queue?: string[] }
+  | { kind: "info"; id: string }
+  | { kind: "person"; name: string }
+  | { kind: "playlist"; id: number };
+
+// Mirror the Plex sub-view to ?v=<kind>:<args> so a show / season / movie-info / person / playlist page
+// is bookmarkable. Grid (root) and the ephemeral player are not mirrored (encode → null). The person
+// name is the whole tail after the first ":", so a name containing ":" round-trips.
+export const PLEX_SUB_URL: SubviewUrl<Sub> = {
+  key: "v",
+  encode: (v) => {
+    switch (v.kind) {
+      case "show":
+        return `show:${v.id}`;
+      case "season":
+        return `season:${v.showId}:${v.seasonId}`;
+      case "info":
+        return `info:${v.id}`;
+      case "person":
+        return `person:${v.name}`;
+      case "playlist":
+        return `playlist:${v.id}`;
+      default:
+        return null; // grid + the ephemeral player are not URL-mirrored
+    }
+  },
+  decode: (raw) => {
+    const i = raw.indexOf(":");
+    const kind = i === -1 ? raw : raw.slice(0, i);
+    const arg = i === -1 ? "" : raw.slice(i + 1);
+    switch (kind) {
+      case "show":
+        return arg ? { kind: "show", id: arg } : null;
+      case "info":
+        return arg ? { kind: "info", id: arg } : null;
+      case "person":
+        return arg ? { kind: "person", name: arg } : null;
+      case "playlist": {
+        // Positive-integer id only — a fractional/zero/negative ?v=playlist:<arg> would open a
+        // sub-view for a non-existent id (matches the ?pl guard in PlaylistsProvider.readSelectedId).
+        const id = arg ? Number(arg) : NaN;
+        return Number.isInteger(id) && id > 0 ? { kind: "playlist", id } : null;
+      }
+      case "season": {
+        const [showId, seasonId] = arg.split(":");
+        return showId && seasonId ? { kind: "season", showId, seasonId } : null;
+      }
+      default:
+        return null;
+    }
+  },
+};