refactor(app): Phase B.2 — split App() into MiniPlayerApp + MainApp (#557)

The 186-LOC default export shrinks to a thin window-kind switch with
shared document-attribute hooks. The mini-player tree and the main-app
tree each move into their own module under src/app/.

  - src/app/MiniPlayerApp.tsx (48 LOC):
      DragDropProvider + MiniPlayer + cross-window storage sync
  - src/app/MainApp.tsx (129 LOC):
      BrowserRouter + Routes + main-only lifecycle hooks
      (audio listeners, hot cache, global shortcuts, mini-player
      bridge, easter egg, scrollbar auto-hide)

AppShell + RequireAuth + TauriEventBridge are now named exports from
App.tsx so MainApp can compose them; Phase C/D will extract those into
their own modules.

App.tsx: 1453 -> 1308 LOC. Behaviour-preserving.

src/app/MiniPlayerApp.test.tsx

@@ -0,0 +1,127 @@
+/**
+ * Smoke + behaviour test for the mini-player webview tree. The component
+ * itself is mostly composition; the meaningful logic is the storage-event
+ * cross-window state-sync handler.
+ */
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+// vi.mock factories run before module-level vars are initialized; route the
+// shared mocks through vi.hoisted() so the references resolve in time.
+const { themeRehydrate, fontRehydrate, keybindingsRehydrate } = vi.hoisted(() => ({
+  themeRehydrate: vi.fn(),
+  fontRehydrate: vi.fn(),
+  keybindingsRehydrate: vi.fn(),
+}));
+
+vi.mock('../store/themeStore', () => ({
+  useThemeStore: { persist: { rehydrate: themeRehydrate } },
+}));
+vi.mock('../store/fontStore', () => ({
+  useFontStore: { persist: { rehydrate: fontRehydrate } },
+}));
+vi.mock('../store/keybindingsStore', () => ({
+  useKeybindingsStore: { persist: { rehydrate: keybindingsRehydrate } },
+}));
+vi.mock('../utils/perfFlags', () => ({
+  usePerfProbeFlags: () => ({ disableTooltipPortal: true }),
+}));
+vi.mock('../i18n', () => ({
+  default: { changeLanguage: vi.fn() },
+}));
+vi.mock('../components/MiniPlayer', () => ({ default: () => <div data-testid="mini-player" /> }));
+vi.mock('../components/GlobalConfirmModal', () => ({ default: () => <div data-testid="confirm-modal" /> }));
+vi.mock('../components/TooltipPortal', () => ({ default: () => <div data-testid="tooltip-portal" /> }));
+vi.mock('../components/FpsOverlay', () => ({ default: () => <div data-testid="fps-overlay" /> }));
+vi.mock('../contexts/DragDropContext', () => ({
+  DragDropProvider: ({ children }: { children: React.ReactNode }) => <>{children}</>,
+}));
+
+import { render, cleanup } from '@testing-library/react';
+import MiniPlayerApp from './MiniPlayerApp';
+import i18n from '../i18n';
+
+beforeEach(() => {
+  themeRehydrate.mockClear();
+  fontRehydrate.mockClear();
+  keybindingsRehydrate.mockClear();
+  vi.mocked(i18n.changeLanguage).mockClear();
+});
+
+afterEach(() => {
+  cleanup();
+});
+
+function fireStorage(key: string | null, newValue: string | null = 'x'): void {
+  // jsdom does not synthesize cross-tab storage events from setItem on the
+  // same window — dispatch one manually.
+  window.dispatchEvent(new StorageEvent('storage', { key: key ?? '', newValue: newValue ?? '' }));
+}
+
+describe('MiniPlayerApp', () => {
+  it('renders the mini-player tree (smoke)', () => {
+    const { getByTestId } = render(<MiniPlayerApp />);
+    expect(getByTestId('mini-player')).toBeTruthy();
+    expect(getByTestId('confirm-modal')).toBeTruthy();
+    expect(getByTestId('fps-overlay')).toBeTruthy();
+  });
+
+  it('hides the tooltip portal when the perf flag disables it', () => {
+    const { queryByTestId } = render(<MiniPlayerApp />);
+    expect(queryByTestId('tooltip-portal')).toBeNull();
+  });
+
+  describe('storage event handler', () => {
+    it('rehydrates themeStore on psysonic_theme writes', () => {
+      render(<MiniPlayerApp />);
+      fireStorage('psysonic_theme');
+      expect(themeRehydrate).toHaveBeenCalledTimes(1);
+      expect(fontRehydrate).not.toHaveBeenCalled();
+    });
+
+    it('rehydrates fontStore on psysonic_font writes', () => {
+      render(<MiniPlayerApp />);
+      fireStorage('psysonic_font');
+      expect(fontRehydrate).toHaveBeenCalledTimes(1);
+    });
+
+    it('rehydrates keybindingsStore on psysonic_keybindings writes', () => {
+      render(<MiniPlayerApp />);
+      fireStorage('psysonic_keybindings');
+      expect(keybindingsRehydrate).toHaveBeenCalledTimes(1);
+    });
+
+    it('switches i18n language on psysonic_language writes', () => {
+      render(<MiniPlayerApp />);
+      fireStorage('psysonic_language', 'de');
+      expect(i18n.changeLanguage).toHaveBeenCalledWith('de');
+    });
+
+    it('ignores psysonic_language when newValue is empty', () => {
+      render(<MiniPlayerApp />);
+      fireStorage('psysonic_language', '');
+      expect(i18n.changeLanguage).not.toHaveBeenCalled();
+    });
+
+    it('ignores unrelated storage keys', () => {
+      render(<MiniPlayerApp />);
+      fireStorage('some-other-key');
+      expect(themeRehydrate).not.toHaveBeenCalled();
+      expect(fontRehydrate).not.toHaveBeenCalled();
+      expect(keybindingsRehydrate).not.toHaveBeenCalled();
+      expect(i18n.changeLanguage).not.toHaveBeenCalled();
+    });
+
+    it('ignores storage events with no key', () => {
+      render(<MiniPlayerApp />);
+      fireStorage(null);
+      expect(themeRehydrate).not.toHaveBeenCalled();
+    });
+
+    it('detaches the listener on unmount', () => {
+      const { unmount } = render(<MiniPlayerApp />);
+      unmount();
+      fireStorage('psysonic_theme');
+      expect(themeRehydrate).not.toHaveBeenCalled();
+    });
+  });
+});