fix(boot): a new tab knows its account before anything renders

With Settings -> Appearance -> Startup turned off, every NEW tab or window still
opened on Home; only a same-tab reload honoured it. The provider tree mounted
before /api/me answered, so NavigationProvider's initializer could not read the
per-account opt-out, landed on Home and mirrored ?page=home into the address bar.
When App pinned the account a moment later, the re-hydrate re-read that mirror
and stayed on Home. The same mount-then-patch shape made a new tab start on
default filters, the default theme and the browser language, and correct each
after the first paint.

BootGate (main.tsx, above the providers) now owns the setup-status and /api/me
queries and mounts the tree only once the identity is settled: it pins the tab's
account and adopts the account's language in a layout effect, so the first frame
of the app is already right. The tree is keyed on the account id, which makes
"per-account state is read once, for this account" a structural guarantee.

With that, the whole re-hydrate machinery is gone: useAccountRehydrate, the
reactive active-account store, the markDirty guards, the pin-deferred saved-view
seed and the present-dimension bookkeeping of the shared Channels/Plex links.
PrefsProvider, FeedFiltersProvider, usePref and App's panel layouts seed from the
me cache instead of adopting it in an effect; the theme is applied in layout
effects. The app chunk is fetched in parallel with the probe, and the boot screen
only fades its label in after 400 ms, on one clock across the gate and the
Suspense fallback.

Module switches no longer flash a placeholder either. Measured frame by frame
in Chromium and WebKit, every first visit to a module showed "Loading..." for
17-70 ms: the lazy chunk's Suspense fallback, then often the first fetch's
LoadingState. Pages are now lazyWithPreload components, and App fetches the
chunks of every module the account can reach (plus the channel page) once the
first page is idle; a preloaded page renders synchronously and never suspends.
LoadingState, the one shared loading visual, fades in only after 300 ms, so a
fast load shows nothing in between and a slow one still gets its spinner.

frontend/src/components/SettingsPanel.tsx

@@ -131,10 +131,9 @@ function Appearance({ prefs }: { prefs: PrefsController }) {
   // Controlled by App's prefs draft: each change applies locally for preview but is only
   // persisted on an explicit Save (handled by the panel's Save/Discard bar).
   const { theme, setTheme, perf, setPerf, hints, setHints } = prefs;
-  // The landing page (E24). NOT a server preference like the rest of this tab: `loadInitialPage`
-  // has to answer before /api/me lands, so the value must be readable synchronously from
-  // localStorage — a landing module decided after the first paint would be a visible jump. The two
-  // stored values come from navigationInit, which is what reads them back at boot.
+  // The landing page (E24). Per-account localStorage, not a server preference like the rest of this
+  // tab: `loadInitialPage` reads it synchronously in NavigationProvider's initializer. The two stored
+  // values come from navigationInit, which is what reads them back at boot.
   const [startOnHome, setStartOnHome] = useAccountPersistedState(LS.startOnHome, START_ON_HOME.on);
 
   return (