refactor: migrate PDF export to desktop runtime abstraction

- Extract desktop-specific logic into src/services/desktop.ts
- Add runtime detection for Electron vs Tauri environments
- Simplify NoteEditor by delegating PDF export to desktop service
- Update printExport.ts to remove unused storage helpers
- Add Electron main process files in electron/ directory
- Update vite config and types for Electron integration
- Update .gitignore and package.json for Electron workflow

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',
+});