feat(nav): full bookmarkable URL model — retire minimalist-URL + Share-view (E4 S2)

The query string is now the source of truth for the current view, so every URL is
bookmarkable/shareable. Reverses the early minimalist-URL rule (user's call).

- urlState: urlWithParams (read-modify-write delta on the live query, each provider
  owns its keys) + clearParams (strip only transient auth params) replace the whole-
  query stripUrlParams; shareUrl deleted; filterParamUpdates helper.
- NavigationProvider: page + ?chan mirrored to the address bar (pushState URL arg;
  history.state mechanics untouched); loadInitialPage resolves deep links
  (?v->plex, ?pl->playlists, filter/?chan->feed, bare->stored landing).
- FeedFiltersProvider: feed filters mirrored to the URL, gated on page===feed.
- useHistorySubview: optional SubviewUrl mapping; PlexBrowse mirrors the sub-view to
  ?v=<kind>:<args> (show/season/info/person/playlist; grid+player stay ephemeral).
- PlaylistsProvider: selected playlist mirrored to ?pl on the Playlists page.
- App: the 5 stripUrlParams calls become targeted clearParams (link/oauth/admin/
  access/verify/reset); the share-link hydrate effect removed.
- Retire the "Share view" feature: Sidebar + SavedViewsWidget buttons + i18n keys.

Browser tab title stays the short module/channel name (document.title untouched) —
the query never leaks into it, per the user's request.

Gate green: typecheck, knip, eslint (0 err), prettier, 247 vitest (urlState.dom.test
+9). No browser-E2E yet (most visual sprint — flagged for the epic-end review).

frontend/src/components/PlexBrowse.tsx

@@ -54,7 +54,7 @@ import {
 import { formatRuntime } from "../lib/format";
 import { notifyActionError } from "../lib/actionErrors";
 import { useDebounced } from "../lib/useDebounced";
-import { useHistorySubview } from "../lib/history";
+import { useHistorySubview, type SubviewUrl } from "../lib/history";
 import {
   DetailCustomizeMenu,
   Filterable,
@@ -83,6 +83,50 @@ type Sub =
   | { 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":
+        return arg && Number.isFinite(Number(arg)) ? { kind: "playlist", id: Number(arg) } : 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
@@ -135,7 +179,7 @@ export default function PlexBrowse() {
   const sortOptions = plexSortOptions(scope, (k) => t(`plex.sort.${k}`));
   // Same idiom for the search scope-pill: a movie-only scope can't hold the Episode field.
   const fieldState = healPlexField(field, scope);
-  const sub = useHistorySubview<Sub>({ kind: "grid" });
+  const sub = useHistorySubview<Sub>({ kind: "grid" }, PLEX_SUB_URL);
 
   // Sidebar asked to open a playlist → push it as a subview (browser Back returns to the grid), then
   // clear the signal so it doesn't re-open.