fix(boot): review round 1 — reconcile the tab pin, one loading clock, honest probe failures

Deep-review round 1 over main..improvement/watch-background-audio.

BootGate
- The tab's account pin is corrected to the account the server answered as.
  The server honours the pin only for an account still signed in to this
  browser and otherwise answers as the default, so a tab pinned to an account
  signed out in another tab ran as the default on the server while reading
  and writing the gone account's local state.
- A mounted tab whose identity changes (its session ended, or the case above)
  reloads instead of rebuilding the tree in place, like every account switch;
  that also drops the previous account's query cache. The account key on the
  tree goes: the readyFor gate already unmounted it on every change.
- An unanswered setup probe shows the error with Retry, like an unanswered
  account probe, instead of the signed-out landing to a signed-in user.
- Comments: /api/me never 401s; the ended-session path is identity → nobody.

Loading states
- LoadingState's reveal delay is one clock across loading states that hand
  over to each other (boot screen → page-chunk fallback → the page's own
  fetching state), instead of a CSS delay restarted by each, which blanked a
  visible spinner for another 300 ms at every hand-off. BootScreen is now the
  shared loading visual, full screen, counting from navigation start.

Plex shared link
- Persists only the dimensions the link carries again. Writing the default
  filters for an omitted `pf` created the stored-filters key, which is what
  tells SavedViewsProvider the account picked filters, so the account's
  default Plex saved view was never applied again.

Watch page
- The Repeat toggle uses the shared pressed-pill paint (a solid fill when on)
  instead of Button variants that differ only in colour, and its flag goes
  through storage.ts readJSON/writeJSON.
- The actions wrap under the title when the row is too narrow, instead of
  squeezing the title into a column a word wide on a phone; the title breaks
  long words.

Comments that still justified code by the removed adopt effects (usePref,
PrefsProvider, FeedViewProvider) are rewritten against the seed-once model.

Tests: BootGate error/retry for both probes, setup mode, a failed background
refetch keeping the app, the pin correction and the reload on an identity
change; the shared reveal clock; the Plex link persist; the audio signature
actually moved off version 4; a video download (av and v) still remuxed to
an mp4. Each mutation-checked RED for its own reason.

frontend/src/components/PlexProvider.tsx

@@ -151,15 +151,25 @@ function readPlexFilters(): PlexFilters {
 }
 // The base is this account's stored view (BootGate pins the account before this provider mounts), so a
 // PARTIAL link — one carrying only the scope, say — keeps the stored sort, show and filters for the rest.
-function readPlexUrlSeed(): PlexUrlState | null {
+// `present` records which dimensions the link actually carried: only those are persisted (below).
+type PlexSeedPresent = { scope: boolean; sort: boolean; show: boolean; filters: boolean };
+type PlexUrlSeed = PlexUrlState & { present: PlexSeedPresent };
+function readPlexUrlSeed(): PlexUrlSeed | null {
   const params = new URLSearchParams(window.location.search);
   if (!hasPlexParams(params)) return null;
-  return plexParamsToState(params, {
+  const state = plexParamsToState(params, {
     scope: readPlexScope(),
     sort: readPlexSort(),
     show: readPlexShow(),
     filters: readPlexFilters(),
   });
+  const present = {
+    scope: params.has("pscope"),
+    sort: params.has("psort"),
+    show: params.has("pshow"),
+    filters: params.has("pf"),
+  };
+  return { ...state, present };
 }
 
 export function PlexProvider({ children }: { children: ReactNode }) {
@@ -195,14 +205,17 @@ export function PlexProvider({ children }: { children: ReactNode }) {
   const [jumpTo, setJumpTo] = useState<PlexJumpTarget | null>(null);
 
   // Persist a shared link's view to the account once, so it wins over the stored view and survives a
-  // later plain reload. The seed's base was the stored view, so a dimension the link omitted is written
-  // back unchanged. Mirrors FeedFiltersProvider's initialHadUrlFilters branch.
+  // later plain reload. Mirrors FeedFiltersProvider's initialHadUrlFilters branch. Only the dimensions
+  // the link CARRIED are written: an omitted one already reads from storage, and writing its default
+  // would not be a no-op — a stored Plex filters value at all is what tells SavedViewsProvider the
+  // account has picked filters, so the account's default saved view would never be applied again.
   useEffect(() => {
     if (!urlSeed) return;
-    setAccountRaw(LS.plexLibrary, urlSeed.scope);
-    setAccountRaw(LS.plexShow, urlSeed.show);
-    setAccountRaw(LS.plexSort, JSON.stringify(urlSeed.sort));
-    setAccountRaw(LS.plexFilters, JSON.stringify(urlSeed.filters));
+    const { present } = urlSeed;
+    if (present.scope) setAccountRaw(LS.plexLibrary, urlSeed.scope);
+    if (present.show) setAccountRaw(LS.plexShow, urlSeed.show);
+    if (present.sort) setAccountRaw(LS.plexSort, JSON.stringify(urlSeed.sort));
+    if (present.filters) setAccountRaw(LS.plexFilters, JSON.stringify(urlSeed.filters));
   }, [urlSeed]);
 
   const setScope = useCallback((v: string) => {