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

src-tauri/capabilities/default.json

@@ -2,9 +2,11 @@
   "$schema": "../gen/schemas/desktop-schema.json",
   "identifier": "default",
   "description": "Capability for the main window",
-  "windows": ["main"],
+  "windows": ["main", "print-export-*"],
   "permissions": [
     "core:default",
+    "core:webview:allow-create-webview-window",
+    "core:webview:allow-print",
     "opener:default",
     "http:default",
     {