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/bootstrap.test.ts

@@ -0,0 +1,145 @@
+/**
+ * Tests for the pre-React bootstrap orchestrator.
+ *
+ * Each step is wrapped in a try/catch so a non-Tauri runtime never breaks
+ * startup; the tests pin both the happy path (invoke called with the right
+ * payload) and the failure-tolerant path (no throw when invoke or storage
+ * misbehaves).
+ */
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+vi.mock('@tauri-apps/api/core', () => ({
+  invoke: vi.fn(),
+}));
+
+vi.mock('../store/playerStore', () => ({
+  installQueueUndoHotkey: vi.fn(),
+}));
+
+vi.mock('./windowKind', () => ({
+  getWindowKind: vi.fn(() => 'main'),
+}));
+
+import { invoke } from '@tauri-apps/api/core';
+import { installQueueUndoHotkey } from '../store/playerStore';
+import { getWindowKind } from './windowKind';
+import {
+  pushLoggingModeToBackend,
+  pushUserAgentToBackend,
+  runPreReactBootstrap,
+} from './bootstrap';
+
+const ORIGINAL_USER_AGENT = window.navigator.userAgent;
+
+function setUserAgent(ua: string): void {
+  Object.defineProperty(window.navigator, 'userAgent', {
+    value: ua,
+    configurable: true,
+  });
+}
+
+beforeEach(() => {
+  vi.clearAllMocks();
+  vi.mocked(invoke).mockResolvedValue(undefined);
+  vi.mocked(getWindowKind).mockReturnValue('main');
+  localStorage.clear();
+  setUserAgent('psysonic-test-ua/1.0');
+});
+
+afterEach(() => {
+  setUserAgent(ORIGINAL_USER_AGENT);
+});
+
+describe('pushUserAgentToBackend', () => {
+  it('forwards the navigator UA to the backend on the main window', () => {
+    pushUserAgentToBackend();
+    expect(invoke).toHaveBeenCalledWith('set_subsonic_wire_user_agent', {
+      userAgent: 'psysonic-test-ua/1.0',
+      windowLabel: 'main',
+    });
+  });
+
+  it('skips the call entirely when the current window is the mini player', () => {
+    vi.mocked(getWindowKind).mockReturnValue('mini');
+    pushUserAgentToBackend();
+    expect(invoke).not.toHaveBeenCalled();
+  });
+
+  it('skips when the navigator UA is empty', () => {
+    setUserAgent('');
+    pushUserAgentToBackend();
+    expect(invoke).not.toHaveBeenCalled();
+  });
+
+  it('does not throw when invoke rejects (non-Tauri runtime)', () => {
+    vi.mocked(invoke).mockRejectedValue(new Error('not tauri'));
+    expect(() => pushUserAgentToBackend()).not.toThrow();
+  });
+});
+
+describe('pushLoggingModeToBackend', () => {
+  it('forwards a valid persisted loggingMode to set_logging_mode', () => {
+    localStorage.setItem(
+      'psysonic-auth',
+      JSON.stringify({ state: { loggingMode: 'debug' } }),
+    );
+    pushLoggingModeToBackend();
+    expect(invoke).toHaveBeenCalledWith('set_logging_mode', { mode: 'debug' });
+  });
+
+  it.each(['off', 'normal', 'debug'])('accepts mode=%s', mode => {
+    localStorage.setItem('psysonic-auth', JSON.stringify({ state: { loggingMode: mode } }));
+    pushLoggingModeToBackend();
+    expect(invoke).toHaveBeenCalledWith('set_logging_mode', { mode });
+  });
+
+  it('skips when no psysonic-auth entry is present', () => {
+    pushLoggingModeToBackend();
+    expect(invoke).not.toHaveBeenCalled();
+  });
+
+  it('skips an unknown mode value', () => {
+    localStorage.setItem(
+      'psysonic-auth',
+      JSON.stringify({ state: { loggingMode: 'verbose' } }),
+    );
+    pushLoggingModeToBackend();
+    expect(invoke).not.toHaveBeenCalled();
+  });
+
+  it('skips when state.loggingMode is missing', () => {
+    localStorage.setItem('psysonic-auth', JSON.stringify({ state: {} }));
+    pushLoggingModeToBackend();
+    expect(invoke).not.toHaveBeenCalled();
+  });
+
+  it('does not throw on malformed JSON', () => {
+    localStorage.setItem('psysonic-auth', '{not json');
+    expect(() => pushLoggingModeToBackend()).not.toThrow();
+    expect(invoke).not.toHaveBeenCalled();
+  });
+});
+
+describe('runPreReactBootstrap', () => {
+  it('runs all steps in order: warm window-kind cache, push UA, push logging mode, install hotkey', () => {
+    localStorage.setItem(
+      'psysonic-auth',
+      JSON.stringify({ state: { loggingMode: 'normal' } }),
+    );
+
+    runPreReactBootstrap();
+
+    expect(getWindowKind).toHaveBeenCalled();
+    expect(invoke).toHaveBeenCalledWith('set_subsonic_wire_user_agent', expect.any(Object));
+    expect(invoke).toHaveBeenCalledWith('set_logging_mode', { mode: 'normal' });
+    expect(installQueueUndoHotkey).toHaveBeenCalledTimes(1);
+  });
+
+  it('still installs the hotkey on the mini window even though UA push is skipped', () => {
+    vi.mocked(getWindowKind).mockReturnValue('mini');
+    runPreReactBootstrap();
+    expect(installQueueUndoHotkey).toHaveBeenCalledTimes(1);
+    // installQueueUndoHotkey itself early-returns inside playerStore on mini —
+    // the bootstrap doesn't second-guess that decision.
+  });
+});