fix(api): a write that hit a connection failure is still reported, and every failure is reported once

Deep-review round 4, the "one report per failure" contract round 3 introduced.

- `req` marked gateway (502/503/504) and network failures "already reported" because they raise
  the connection notice. But that notice is about connectivity and removes itself on the very next
  answer — which every caller's own resync refetch fetches at once — so a delete that hit a
  keepalive reset flashed the notice and was then reported nowhere, the list still there. Only the
  durable reports (the error dialog, the 401 sign-out) count now; the caller says the action failed.
- A connection notice the user closed swallowed every later one for the rest of the visit; it is
  raised again.
- `req` and `uploadReq` share one failure policy (`reportFailure`). The upload copy had drifted: a
  502 opened the blocking "Server error (502)" dialog and cleared a notice that was still true.
- A taken Plex collection name is a structured 409, localized, and the create/rename calls are
  quiet: it was an English dialog plus a Hungarian toast. `notifyCollectionEditError` is gone.
- The mutation onErrors that toasted unconditionally after a non-quiet request (invite approve /
  deny / add, demo whitelist, audit clear, scheduler, share, full history, edit, config tests, tag
  attach) go through `notifyActionError`; the playlist delete's hand-rolled copy of it too (it takes
  a level now). The user role / suspend / delete actions had no onError at all: a user deleted in
  another tab failed silently and stayed listed — now said, and the list refetched.

frontend/src/lib/api/plex.ts

@@ -505,15 +505,19 @@ export const plexApi = {
     return req(`/api/plex/collections/${encodeURIComponent(rk)}${qs ? `?${qs}` : ""}`);
   },
   // --- Collection editing (admin only; writes back to Plex) ---
+  // `quiet`: the editor reports a failure itself, as a toast beside the name being typed (a taken
+  // name comes back structured and localized — see localizeDetail). Not quiet, it was reported twice:
+  // the global error dialog in English AND the editor's toast.
   plexCreateCollection: (
     library: string,
     title: string,
     item_rating_key: string,
   ): Promise<PlexCollection> =>
-    req(`/api/plex/collections`, {
-      method: "POST",
-      body: JSON.stringify({ library, title, item_rating_key }),
-    }),
+    req(
+      `/api/plex/collections`,
+      { method: "POST", body: JSON.stringify({ library, title, item_rating_key }) },
+      { quiet: true },
+    ),
   plexCollectionAddItem: (rk: string, itemRk: string): Promise<PlexCollection> =>
     req(`/api/plex/collections/${encodeURIComponent(rk)}/items/${encodeURIComponent(itemRk)}`, {
       method: "POST",
@@ -523,10 +527,11 @@ export const plexApi = {
       method: "DELETE",
     }),
   plexRenameCollection: (rk: string, title: string): Promise<PlexCollection> =>
-    req(`/api/plex/collections/${encodeURIComponent(rk)}`, {
-      method: "PATCH",
-      body: JSON.stringify({ title }),
-    }),
+    req(
+      `/api/plex/collections/${encodeURIComponent(rk)}`,
+      { method: "PATCH", body: JSON.stringify({ title }) },
+      { quiet: true }, // as plexCreateCollection
+    ),
   plexDeleteCollection: (rk: string): Promise<{ deleted: string }> =>
     req(`/api/plex/collections/${encodeURIComponent(rk)}`, { method: "DELETE" }),
   plexSetCollectionEditable: (rk: string, editable: boolean): Promise<PlexCollection> =>