fix(api): a gone row and a refused playlist action are said in the reader's language, and a failure is reported once

Deep-review round 3 of bug hunt 2026-09.

- get_or_404 / owned_or_404 answer a structured {code: "not_found", reason: <what>}
  (errors.notFound.* in HU+EN, generic fallback). The structured-404 fix of round 2 had gone into the
  ad-hoc `_own_playlist` (CLAUDE.md's "Not" pattern) while the shared helpers stayed string-only, so
  views, tags, pins, Plex playlists, invites and users still 404'd in English. `_own_playlist` now
  goes through owned_or_404; admin's hand-rolled user lookups use get_or_404.
- A move into a list that is gone names the TARGET (reason "move_target") — "this playlist doesn't
  exist any more" was read as the list on screen, which still exists.
- Every refusal in the playlists router is a {code: "playlist", reason} (playlists.errors.* HU+EN)
  instead of a bare English string shown verbatim in the Hungarian UI.
- `req` / `uploadReq` mark the errors they already put on screen (the error dialog, the connection
  notice, the 401 sign-out), and notifyActionError skips those. The watched toggles, the saved views
  and a playlist delete toasted a fault the dialog was already showing; Playlists' two hand-written
  status filters now go through the same check.

frontend/src/lib/actionErrors.test.ts

@@ -6,9 +6,9 @@ import { describe, it, expect, vi, beforeEach } from "vitest";
 // Spy on the toast sink so we can assert exactly what each error maps to.
 vi.mock("./notifications", () => ({ notify: vi.fn(() => 0) }));
 
-import { HttpError } from "./api";
+import { HttpError, api } from "./api";
 import { notify } from "./notifications";
-import { notifyCollectionEditError } from "./actionErrors";
+import { notifyActionError, notifyCollectionEditError } from "./actionErrors";
 
 const notifyMock = vi.mocked(notify);
 beforeEach(() => notifyMock.mockClear());
@@ -29,3 +29,25 @@ describe("notifyCollectionEditError", () => {
     expect(notifyMock).toHaveBeenCalledWith({ level: "error", message: "EDIT_FAILED" });
   });
 });
+
+describe("notifyActionError", () => {
+  it("does not toast a fault the request layer already put on screen", async () => {
+    // A 500 opens the blocking error dialog inside `req`; the watched toggles, the saved views and a
+    // playlist delete then toasted the same failure as well.
+    vi.spyOn(globalThis, "fetch").mockResolvedValue(
+      new Response(JSON.stringify({ detail: "boom" }), {
+        status: 500,
+        headers: { "content-type": "application/json" },
+      }),
+    );
+    const err = await api.plexSetState("1", "watched").catch((e: unknown) => e);
+    notifyMock.mockClear();
+    notifyActionError(err, "FAILED");
+    expect(notifyMock).not.toHaveBeenCalled();
+  });
+
+  it("does toast one the caller was left to handle", () => {
+    notifyActionError(new HttpError(404, "gone"), "FAILED");
+    expect(notifyMock).toHaveBeenCalledWith({ level: "error", message: "gone" });
+  });
+});