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/components/Channels.tsx

@@ -6,6 +6,7 @@ import { Ban, Eye, EyeOff, History, LayoutGrid, Pencil, RefreshCw, Table, X } fr
 import ExcludedChannelList from "./ExcludedChannelList";
 import { api, NO_TAG_SENTINEL, type ManagedChannel } from "../lib/api";
 import { Sparkles } from "lucide-react";
+import { notifyActionError } from "../lib/actionErrors";
 import { notifyYouTubeActionError } from "../lib/youtubeErrors";
 import { accountKey, LS } from "../lib/storage";
 import { buildChannelColumns, applySyncFilter } from "./channelColumns";
@@ -182,10 +183,10 @@ export default function Channels() {
     // suggestions: acceptSuggestion optimistically dropped the channel from that cache, and on
     // failure the tag never actually attached — so the server still suggests it and the ghost chips
     // must come back rather than leaving an empty cell (honest states). Surface the write failure too.
-    onError: () => {
+    onError: (e) => {
       qc.invalidateQueries({ queryKey: qk.channels() });
       qc.invalidateQueries({ queryKey: qk.tagSuggestions() });
-      notify({ level: "error", message: t("channels.tags.updateFailed") });
+      notifyActionError(e, t("channels.tags.updateFailed"));
     },
   });
   // --- Suggested tags (E8·S4): learned from the user's own manual-tag pattern, shown as accept/dismiss