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/actionErrors.ts

@@ -10,30 +10,15 @@ import { notify } from "./notifications";
  * the Plex watched-toggles and the saved-view ops, and delegated to by `notifyYouTubeActionError`
  * for its non-403 case.
  */
-export function notifyActionError(err: unknown, fallbackMessage: string): void {
-  // Already on screen (`req` opened the error dialog, or raised the connection notice): a toast on
+export function notifyActionError(
+  err: unknown,
+  fallbackMessage: string,
+  level: "error" | "warning" = "error",
+): void {
+  // Already on screen (`req` opened the error dialog, or the tab is being signed out): a toast on
   // top reported one fault twice. Playlists filtered this by status by hand; the watched toggles,
   // the saved views and a playlist delete did not.
   if (alreadyReported(err)) return;
   const detail = err instanceof HttpError ? err.detail : null;
-  notify({ level: "error", message: detail || fallbackMessage });
-}
-
-/**
- * Collection write errors. The backend enforces per-library name uniqueness and returns 409 on a
- * duplicate; localize that specific case to the friendly `nameTaken` copy (notifyActionError would
- * otherwise surface the server's raw English detail). Everything else falls back to the generic
- * edit-failed handling (which still prefers the server's reason when present). Both args are already
- * translated so this stays i18n-agnostic and unit-testable.
- */
-export function notifyCollectionEditError(
-  err: unknown,
-  editFailedMessage: string,
-  nameTakenMessage: string,
-): void {
-  if (err instanceof HttpError && err.status === 409) {
-    notify({ level: "error", message: nameTakenMessage });
-    return;
-  }
-  notifyActionError(err, editFailedMessage);
+  notify({ level, message: detail || fallbackMessage });
 }