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/BootGate.dom.test.tsx

@@ -0,0 +1,93 @@
+// @vitest-environment jsdom
+//
+// BootGate settles the tab's identity BEFORE the app tree mounts. The bug it replaced: a new tab (no
+// pinned account in sessionStorage) built the app for "no account" first — so with Settings →
+// Appearance → Startup turned off, NavigationProvider's initializer could not read the opt-out and
+// landed on Home, mirrored ?page=home into the address bar, and then re-read its own mirror when the
+// account was pinned, staying on Home. Only a same-tab reload (account already pinned) was right.
+//
+// Real NavigationProvider, real storage and i18n; only the two boot probes are stubbed. The probe
+// records EVERY render, so "never showed the wrong page, not even for one render" is what is asserted.
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { render, screen, cleanup, act } from "@testing-library/react";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import { useTranslation } from "react-i18next";
+import BootGate from "./BootGate";
+import { NavigationProvider, useNavigation } from "./NavigationProvider";
+import { api, type Me } from "../lib/api";
+import { ACTIVE_ACCOUNT_KEY } from "../lib/activeAccount";
+import i18n from "../i18n";
+
+const renders: { page: string; lang: string }[] = [];
+function Probe() {
+  const { page } = useNavigation();
+  const { i18n: inst } = useTranslation();
+  renders.push({ page, lang: inst.language });
+  return <div data-testid="app">{page}</div>;
+}
+
+function mount() {
+  const qc = new QueryClient({ defaultOptions: { queries: { retry: false } } });
+  render(
+    <QueryClientProvider client={qc}>
+      <BootGate>
+        <NavigationProvider>
+          <Probe />
+        </NavigationProvider>
+      </BootGate>
+    </QueryClientProvider>,
+  );
+}
+const account = (over: Partial<Me> = {}) => ({ id: 5, preferences: {}, ...over }) as Me;
+
+beforeEach(async () => {
+  localStorage.clear();
+  sessionStorage.clear(); // a NEW tab: nothing pinned yet
+  window.history.replaceState(null, "", "/");
+  renders.length = 0;
+  await i18n.changeLanguage("en");
+  vi.spyOn(api, "setupStatus").mockResolvedValue({ configured: true });
+});
+afterEach(() => {
+  cleanup();
+  vi.restoreAllMocks();
+});
+
+describe("BootGate", () => {
+  it("a new tab with Startup off opens on the page the account left — never on Home first", async () => {
+    localStorage.setItem("siftlode.startOnHome.5", "0");
+    localStorage.setItem("siftlode.page.5", "downloads");
+    vi.spyOn(api, "me").mockResolvedValue(account());
+    mount();
+
+    expect((await screen.findByTestId("app")).textContent).toBe("downloads");
+    expect(renders.map((r) => r.page)).toEqual(renders.map(() => "downloads"));
+    expect(new URLSearchParams(window.location.search).get("page")).toBe("downloads");
+    expect(sessionStorage.getItem(ACTIVE_ACCOUNT_KEY)).toBe("5"); // the tab is pinned
+  });
+
+  it("renders the app in the account's language from its first render", async () => {
+    vi.spyOn(api, "me").mockResolvedValue(account({ preferences: { language: "hu" } }));
+    mount();
+
+    await screen.findByTestId("app");
+    expect(renders.map((r) => r.lang)).toEqual(renders.map(() => "hu"));
+  });
+
+  it("mounts nothing of the app while the identity is still unknown", async () => {
+    vi.spyOn(api, "me").mockReturnValue(new Promise<Me | null>(() => {}));
+    mount();
+    await act(async () => {}); // the setup probe resolves; `me` never does
+
+    expect(screen.queryByTestId("app")).toBeNull();
+    expect(renders).toHaveLength(0);
+  });
+
+  it("signed out: mounts the app (App shows the landing) without pinning an account", async () => {
+    vi.spyOn(api, "me").mockResolvedValue(null);
+    mount();
+
+    await screen.findByTestId("app");
+    expect(sessionStorage.getItem(ACTIVE_ACCOUNT_KEY)).toBeNull();
+  });
+});