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.ts

@@ -0,0 +1,30 @@
+import { getCurrentWindow } from '@tauri-apps/api/window';
+
+export type WindowKind = 'main' | 'mini';
+
+let cached: WindowKind | null = null;
+
+/**
+ * Tauri window-label detection, cached after the first call. The result
+ * decides whether App() renders the full main UI tree or the standalone
+ * mini-player tree, and it must be consistent for the lifetime of the
+ * webview — Tauri never changes a window's label.
+ *
+ * Falls back to 'main' in non-Tauri environments (jsdom, plain browser).
+ */
+export function getWindowKind(): WindowKind {
+  if (cached !== null) return cached;
+  let label: string;
+  try {
+    label = getCurrentWindow().label;
+  } catch {
+    label = 'main';
+  }
+  cached = label === 'mini' ? 'mini' : 'main';
+  return cached;
+}
+
+/** Test-only: clears the cached window kind so each test starts clean. */
+export function _resetWindowKindCacheForTest(): void {
+  cached = null;
+}