Migrate to Electron and implement native PDF export

Major changes:
- Migrate from Tauri to Electron as primary desktop runtime
- Implement native print dialog for PDF export via Electron webview
- Add desktop runtime abstraction layer (supports both Electron and Tauri)
- Implement task list rendering in preview mode
- Add favorite notes sorting to display starred notes at top
- Add attachment upload functionality with file picker
- Improve sync reliability and Unicode filename support
- Add category color sync across devices via WebDAV
- Update documentation for Electron workflow

Technical improvements:
- Add Electron main process and preload bridge
- Create desktop service layer for runtime-agnostic operations
- Implement runtimeFetch for proxying network requests through Electron
- Add PrintView component for native print rendering
- Extract print/PDF utilities to shared module
- Update build configuration for Electron integration

vite.config.ts

@@ -5,8 +5,9 @@ import react from "@vitejs/plugin-react";
 const host = process.env.TAURI_DEV_HOST;
 
 // https://vite.dev/config/
-export default defineConfig(async () => ({
+export default defineConfig(async ({ command }) => ({
   plugins: [react()],
+  base: command === "build" ? "./" : "/",
 
   // Vite options tailored for Tauri development and only applied in `tauri dev` or `tauri build`
   //
@@ -17,6 +18,10 @@ export default defineConfig(async () => ({
     port: 1420,
     strictPort: true,
     host: host || false,
+    headers: {
+      "Content-Security-Policy":
+        "default-src 'self'; script-src 'self' 'unsafe-inline'; style-src 'self' 'unsafe-inline'; img-src 'self' data: blob: https: http:; font-src 'self' data:; connect-src 'self' https: http: ws: wss:; object-src 'none'; base-uri 'self'; frame-ancestors 'none'",
+    },
     hmr: host
       ? {
           protocol: "ws",