refactor(app): Phase B.1 — extract pre-React bootstrap into src/app/ (#555)

main.tsx shrinks from 56 -> 17 LOC. New module surface:

  - src/app/windowKind.ts: cached getWindowKind() detector,
    replaces the global __PSY_WINDOW_LABEL__ string everywhere
  - src/app/bootstrap.ts: pushUserAgentToBackend +
    pushLoggingModeToBackend + runPreReactBootstrap orchestrator

App.tsx + playerStore.ts now read getWindowKind() instead of poking
window.__PSY_WINDOW_LABEL__ directly. Behaviour-preserving.

src/app/windowKind.test.ts

@@ -0,0 +1,66 @@
+/**
+ * Tests for the cached Tauri window-kind detector.
+ *
+ * The cache is module-scoped, so each test must reset it via the
+ * `_resetWindowKindCacheForTest()` escape hatch — otherwise the first call
+ * locks the value for the rest of the file.
+ */
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+vi.mock('@tauri-apps/api/window', () => ({
+  getCurrentWindow: vi.fn(),
+}));
+
+import { getCurrentWindow } from '@tauri-apps/api/window';
+import { _resetWindowKindCacheForTest, getWindowKind } from './windowKind';
+
+beforeEach(() => {
+  _resetWindowKindCacheForTest();
+  vi.clearAllMocks();
+});
+
+afterEach(() => {
+  _resetWindowKindCacheForTest();
+});
+
+describe('getWindowKind', () => {
+  it('returns "main" when the current window label is "main"', () => {
+    vi.mocked(getCurrentWindow).mockReturnValue({ label: 'main' } as ReturnType<typeof getCurrentWindow>);
+    expect(getWindowKind()).toBe('main');
+  });
+
+  it('returns "mini" when the current window label is "mini"', () => {
+    vi.mocked(getCurrentWindow).mockReturnValue({ label: 'mini' } as ReturnType<typeof getCurrentWindow>);
+    expect(getWindowKind()).toBe('mini');
+  });
+
+  it('falls back to "main" for any other label', () => {
+    vi.mocked(getCurrentWindow).mockReturnValue({ label: 'something-else' } as ReturnType<typeof getCurrentWindow>);
+    expect(getWindowKind()).toBe('main');
+  });
+
+  it('falls back to "main" when getCurrentWindow throws (non-Tauri runtime)', () => {
+    vi.mocked(getCurrentWindow).mockImplementation(() => {
+      throw new Error('not in tauri');
+    });
+    expect(getWindowKind()).toBe('main');
+  });
+
+  it('caches the first result and does not call getCurrentWindow again', () => {
+    vi.mocked(getCurrentWindow).mockReturnValue({ label: 'mini' } as ReturnType<typeof getCurrentWindow>);
+    expect(getWindowKind()).toBe('mini');
+    expect(getWindowKind()).toBe('mini');
+    expect(getWindowKind()).toBe('mini');
+    expect(getCurrentWindow).toHaveBeenCalledTimes(1);
+  });
+
+  it('re-reads after the cache is reset', () => {
+    vi.mocked(getCurrentWindow).mockReturnValue({ label: 'main' } as ReturnType<typeof getCurrentWindow>);
+    expect(getWindowKind()).toBe('main');
+
+    _resetWindowKindCacheForTest();
+    vi.mocked(getCurrentWindow).mockReturnValue({ label: 'mini' } as ReturnType<typeof getCurrentWindow>);
+    expect(getWindowKind()).toBe('mini');
+    expect(getCurrentWindow).toHaveBeenCalledTimes(2);
+  });
+});