fix(plex,channels): deep-review fixes — durable watch-sync + no shared-link clobber

Deep-review of the quickfix batch surfaced two data/preference-loss bugs plus
smaller items; all fixed with tests.

- watch_sync _pull_apply: don't let the Plex-authoritative pull overwrite a DIRTY
  local row (a change whose immediate push failed) — that stranded the value
  forever since _repush_dirty only retries dirty rows. Leave it dirty for re-push.
- PlexProvider / ChannelsProvider: a partial shared link opened in a fresh tab
  persisted mount-time DEFAULTS over the account's stored scope/sort/filters
  (base read before the account was pinned). Persist/keep only URL-present dims;
  re-hydrate the omitted ones from storage.
- playback resume write-back: guard against out-of-order delivery (a superseded
  older push now drops) and evict the throttle map so it can't grow unbounded.
- PlexPlayer: announce mid-playback buffering to assistive tech (role=status +
  sr-only label); new plex.player.buffering (HU+EN).
- columnSort: hoist the shared key:dir sort codec used by both URL-state modules.
- Tests: dirty-guard + flag-settle + per-key throttle scoping + superseded-push
  (backend); hold-repeat/suppress-click/unmount + partial-link no-clobber +
  refetch-doesn't-clobber-undo (frontend). Backend pytest 475, FE vitest 519.

frontend/src/i18n/locales/en/plex.json

@@ -154,6 +154,7 @@
   },
   "player": {
     "loading": "Loading…",
+    "buffering": "Buffering…",
     "back": "Back",
     "skipIntro": "Skip intro",
     "skipCredits": "Skip credits",

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

@@ -154,6 +154,7 @@
   },
   "player": {
     "loading": "Betöltés…",
+    "buffering": "Pufferelés…",
     "back": "Vissza",
     "skipIntro": "Intro átugrása",
     "skipCredits": "Stáblista átugrása",