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/i18n/locales/en/users.json

@@ -39,7 +39,8 @@
     "promoteTitle": "Make admin?",
     "demoteTitle": "Remove admin?",
     "promoteConfirm": "Give {{email}} full admin access (settings, scheduler, user management)?",
-    "demoteConfirm": "Remove admin access from {{email}}? They'll become a regular user."
+    "demoteConfirm": "Remove admin access from {{email}}? They'll become a regular user.",
+    "failed": "Couldn't change the role."
   },
   "suspend": {
     "action": "Suspend",
@@ -51,14 +52,16 @@
     "done": "{{email}} suspended",
     "undone": "{{email}} unsuspended",
     "demoLocked": "The demo account can't be suspended.",
-    "selfLocked": "You can't suspend your own account."
+    "selfLocked": "You can't suspend your own account.",
+    "failed": "Couldn't change the suspension."
   },
   "delete": {
     "action": "Delete",
     "locked": "This account can't be deleted here.",
     "title": "Delete account?",
     "confirm": "Permanently delete {{email}} and all their data — subscriptions, tags, watch history, playlists and settings? This can't be undone.",
-    "done": "{{email}} deleted"
+    "done": "{{email}} deleted",
+    "failed": "Couldn't delete the account."
   },
   "account": {
     "access": "Access",

frontend/src/i18n/locales/hu/users.json

@@ -39,7 +39,8 @@
     "promoteTitle": "Adminná teszed?",
     "demoteTitle": "Elveszed az admin jogot?",
     "promoteConfirm": "Teljes admin hozzáférést adsz neki: {{email}} (beállítások, ütemező, felhasználókezelés)?",
-    "demoteConfirm": "Elveszed az admin jogot tőle: {{email}}? Sima felhasználó lesz."
+    "demoteConfirm": "Elveszed az admin jogot tőle: {{email}}? Sima felhasználó lesz.",
+    "failed": "Nem sikerült módosítani a szerepkört."
   },
   "suspend": {
     "action": "Felfüggeszt",
@@ -51,14 +52,16 @@
     "done": "{{email}} felfüggesztve",
     "undone": "{{email}} felfüggesztése feloldva",
     "demoLocked": "A demo fiók nem függeszthető fel.",
-    "selfLocked": "A saját fiókodat nem függesztheted fel."
+    "selfLocked": "A saját fiókodat nem függesztheted fel.",
+    "failed": "Nem sikerült módosítani a felfüggesztést."
   },
   "delete": {
     "action": "Törlés",
     "locked": "Ez a fiók itt nem törölhető.",
     "title": "Törlöd a fiókot?",
     "confirm": "Véglegesen törlöd: {{email}} és minden adatát — feliratkozások, címkék, megtekintési előzmény, lejátszási listák és beállítások? Ez nem visszavonható.",
-    "done": "{{email}} törölve"
+    "done": "{{email}} törölve",
+    "failed": "Nem sikerült törölni a fiókot."
   },
   "account": {
     "access": "Hozzáférés",