feat(phase1): architecture skeleton

- Electron 33 main process with frameless window, IPC, win-state persistence
- Preload with contextBridge typed API (window, theme, dialog)
- Vue 3 + Pinia + Element Plus + Vue Router renderer
- Dark/light dual theme via CSS custom properties
- TitleBar with window controls + theme toggle
- StatusBar placeholder
- Design spec document

src/preload/index.d.ts

@@ -0,0 +1,23 @@
+// src/preload/index.d.ts
+export interface ElectronAPI {
+  window: {
+    minimize: () => void
+    maximize: () => void
+    close: () => void
+  }
+  theme: {
+    get: () => Promise<boolean>
+    set: (theme: 'system' | 'dark' | 'light') => void
+    onOsUpdated: (callback: (isDark: boolean) => void) => void
+  }
+  dialog: {
+    openFile: (options: { title?: string; filters?: { name: string; extensions: string[] }[] }) =>
+      Promise<{ path: string; content: string } | null>
+  }
+}
+
+declare global {
+  interface Window {
+    electronAPI: ElectronAPI
+  }
+}