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.

backend/app/routes/plex/collections.py

@@ -347,7 +347,9 @@ def _reject_duplicate_name(
         .first()
     )
     if dup is not None and dup.rating_key != exclude_rk:
-        raise HTTPException(status_code=409, detail="A collection with this name already exists")
+        # Structured, so the bilingual UI says it in the reader's language (it was toasted in
+        # Hungarian beside an English error dialog saying the same thing).
+        raise HTTPException(status_code=409, detail={"code": "collection", "reason": "name_taken"})
 
 
 def _write_or_pending_error(wrote: bool, e: Exception) -> HTTPException:

backend/tests/test_plex_collections_filter.py

@@ -155,6 +155,7 @@ def test_create_collection_rejects_duplicate_name(db, monkeypatch):
             json={"library": "1", "title": "marvel", "item_rating_key": "m1"},
         )
         assert r.status_code == 409, r.text
+        assert r.json()["detail"] == {"code": "collection", "reason": "name_taken"}
     finally:
         app.dependency_overrides.clear()