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/ConfigPanel.tsx

@@ -14,6 +14,7 @@ import { PageToolbar } from "./ui/PageShell";
 import { Switch } from "./ui/form";
 import { DraftSaveBar, type SaveState } from "./ui/DraftSaveBar";
 import { LS } from "../lib/storage";
+import { notifyActionError } from "../lib/actionErrors";
 
 // Admin Configuration page: edit DB-overridable operational settings (registry-driven from the
 // backend — adding a key there makes it appear here automatically). Edits are drafted and
@@ -125,7 +126,7 @@ export default function ConfigPanel() {
   const testEmail = useMutation({
     mutationFn: () => api.testEmail(),
     onSuccess: (r) => notify({ level: "success", message: t("config.testSent", { to: r.to }) }),
-    onError: () => notify({ level: "error", message: t("config.testFailed") }),
+    onError: (e) => notifyActionError(e, t("config.testFailed")),
   });
 
   // Plex "Test connection": verifies the server URL + token and returns the library sections,
@@ -138,9 +139,9 @@ export default function ConfigPanel() {
       setPlexResult(r);
       notify({ level: "success", message: t("config.plexTestOk", { name: r.server_name }) });
     },
-    onError: () => {
+    onError: (e) => {
       setPlexResult(null);
-      notify({ level: "error", message: t("config.plexTestFailed") });
+      notifyActionError(e, t("config.plexTestFailed"));
     },
   });
   const plexLibs = (draft["plex_libraries"] ?? "")