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

electron/preload.cjs

@@ -0,0 +1,8 @@
+const { contextBridge, ipcRenderer } = require('electron');
+
+contextBridge.exposeInMainWorld('electronDesktop', {
+  showMessage: (options) => ipcRenderer.invoke('desktop:show-message', options),
+  exportPdf: (payload) => ipcRenderer.invoke('desktop:export-pdf', payload),
+  httpRequest: (payload) => ipcRenderer.invoke('desktop:http-request', payload),
+  getRuntime: () => 'electron',
+});