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/lib/storage.ts

@@ -65,7 +65,7 @@ export const LS = {
   notifSettings: "siftlode.notifSettings",
   onboardingDismissed: "siftlode.onboarding.dismissed",
   seenVersion: "siftlode.seenVersion",
-  // The public /watch page's repeat toggle ("0" = off; on by default). Browser-wide, NOT per account:
+  // The public /watch page's repeat toggle (JSON boolean; on by default). Browser-wide, NOT per account:
   // that page runs outside the app, with no account at all.
   watchRepeat: "siftlode.watchRepeat",
   chatDock: (userId: number) => `siftlode.chatDock.${userId}`,

frontend/src/lib/usePref.ts

@@ -20,8 +20,8 @@ const reportPrefSaveFailure = (err: unknown) =>
 // server-synced pref is a one-liner.
 //
 // It seeds straight from the `me` cache, which is always populated when it mounts: BootGate mounts the
-// app tree only once /api/me has answered, keyed on the account, so there is nothing to adopt after
-// mount. The source of truth is the server; the `me` cache is kept in sync optimistically so other
+// app tree only once /api/me has answered, and a tab whose account changes reloads, so there is nothing
+// to adopt after mount. The source of truth is the server; the `me` cache is kept in sync optimistically so other
 // consumers reading the same key (e.g. a second player instance) see the change at once.
 
 export interface UsePrefOptions<T> {
@@ -75,8 +75,8 @@ export function usePref<T>(
   const { localKey, debounceMs, parse } = options;
   const qc = useQueryClient();
 
-  // Latest parse in a ref so the seed/adopt/setter don't depend on a fresh closure each render (which
-  // would churn the memoized setter and the adopt effect). parse must be idempotent anyway.
+  // Latest parse in a ref so the seed and the setter don't depend on a fresh closure each render (which
+  // would churn the memoized setter). parse must be idempotent anyway.
   const parseRef = useRef(parse);
   parseRef.current = parse;
   const norm = useCallback((raw: unknown): T => {
@@ -98,7 +98,7 @@ export function usePref<T>(
   const saveTimer = useRef<number | undefined>(undefined);
   // The pending debounced server write, if any — flushed (NOT dropped) on unmount so the last change
   // isn't lost. Dropping it would leave the server stale while local/cache/localStorage hold the new
-  // value, and a later adopt could then overwrite the newer local value with the stale server one.
+  // value, and the next load — which seeds from the server first — would bring the old value back.
   const pendingSave = useRef<(() => void) | null>(null);
   useEffect(
     () => () => {
@@ -113,8 +113,8 @@ export function usePref<T>(
     (next: T) => {
       setValue(next);
       if (localKey) writeAccount(localKey, next);
-      // Keep the `me` cache in step so other readers of this key (and this hook's own adopt effect)
-      // see the new value without a refetch. Merges into preferences, mirroring the server PUT.
+      // Keep the `me` cache in step so other readers of this key (and a later mount of this hook,
+      // which seeds from the cache) see the new value without a refetch. Merges into preferences, mirroring the server PUT.
       const payload = { [serverKey]: next };
       mergePrefsCache(qc, payload);
       // savePrefs is fire-and-forget (idempotent PUT, so `req` retries a transient blip once); a