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/PrefsProvider.tsx

@@ -28,8 +28,8 @@ const PERF_KEY = LS.perfMode;
 
 // The four Settings-page preferences. They apply locally for instant preview and auto-save (debounced)
 // to the server — there is no draft or Save button. This provider owns them (lifted out of the App god
-// component) plus the last-saved "baseline" used to suppress the echo when a change came from adopting
-// server prefs on login. Consumers (SettingsPanel, GlassTuner) read the PrefsController via usePrefs().
+// component) plus the last-saved "baseline" (the server values it was seeded with, then each successful
+// save) used to suppress saving what the server already holds. Consumers (SettingsPanel, GlassTuner) read the PrefsController via usePrefs().
 type EditablePrefs = {
   theme: ThemePrefs;
   performanceMode: boolean;
@@ -102,8 +102,8 @@ export function PrefsProvider({ children }: { children: ReactNode }) {
   useEffect(() => configureNotifications(notif), [notif]);
 
   // Auto-save on change (debounced ~500ms so a slider drag coalesces into one PUT). Live-apply is the
-  // effects above; this is the only persistence path. `baselineRef` suppresses the echo when the
-  // change came from adopting server prefs on login (`/me` merges, so a full payload is fine).
+  // effects above; this is the only persistence path. `baselineRef` keeps the mount seed from being
+  // saved straight back (`/me` merges, so a full payload is fine).
   const editablePrefs = useMemo<EditablePrefs>(
     () => ({ theme, performanceMode: perf, hints, notifications: notif }),
     [theme, perf, hints, notif],
@@ -111,7 +111,7 @@ export function PrefsProvider({ children }: { children: ReactNode }) {
   useEffect(() => {
     window.clearTimeout(saveTimer.current);
     if (JSON.stringify(editablePrefs) === JSON.stringify(baselineRef.current)) {
-      // Back at the last-saved value (initial load, server adopt, or an edit undone) — nothing to
+      // Back at the last-saved value (the mount seed, or an edit undone) — nothing to
       // save; clear any "Saving…" left from a change that was reverted before it flushed.
       setSaveState((s) => (s === "saving" ? "idle" : s));
       return;