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.ts

@@ -0,0 +1,45 @@
+import { invoke } from '@tauri-apps/api/core';
+import { installQueueUndoHotkey } from '../store/playerStore';
+import { getWindowKind } from './windowKind';
+
+/** Sync backend HTTP User-Agent from the main webview once at startup. */
+export function pushUserAgentToBackend(): void {
+  try {
+    if (getWindowKind() !== 'main') return;
+    const ua = window.navigator.userAgent?.trim();
+    if (ua) {
+      void invoke('set_subsonic_wire_user_agent', { userAgent: ua, windowLabel: 'main' });
+    }
+  } catch {
+    // Ignore in non-Tauri runtimes.
+  }
+}
+
+/**
+ * Push the persisted logging mode to Rust before React mounts. Zustand rehydrate
+ * runs after first paint; AppShell's useEffect can miss the user's persisted
+ * `loggingMode` until then — but waveform/audio may already run. Matches the
+ * `psysonic-auth` localStorage key.
+ */
+export function pushLoggingModeToBackend(): void {
+  try {
+    const raw = localStorage.getItem('psysonic-auth');
+    if (!raw) return;
+    const parsed = JSON.parse(raw) as { state?: { loggingMode?: string } };
+    const mode = parsed.state?.loggingMode;
+    if (mode === 'off' || mode === 'normal' || mode === 'debug') {
+      void invoke('set_logging_mode', { mode });
+    }
+  } catch {
+    // Ignore parse / non-Tauri.
+  }
+}
+
+/** Orchestrates everything that must run before React mounts. */
+export function runPreReactBootstrap(): void {
+  // Pre-warm the window-kind cache so subsequent reads are sync + safe.
+  getWindowKind();
+  pushUserAgentToBackend();
+  pushLoggingModeToBackend();
+  installQueueUndoHotkey();
+}