fix: platform selector + notifications (Round 21)

[wallydz-bot]

Fixes two critical bugs

Bug 1: App opened messenger.com directly instead of platform selector

• Root cause: dist/index.html had hardcoded window.location.href = "https://www.messenger.com" — the TypeScript frontend was never set up with a build system
• Fix: Added Vite build pipeline (vite.config.ts, updated package.json scripts). Frontend now compiles from src/ TypeScript to dist/
• dist/index.html now serves the platform selector with bundled JS at dist/assets/main-*.js

Bug 2: Notifications broken on Windows / Mac / Linux

Three simultaneous failures:

#	Root Cause	Fix
1	notification-interceptor.ts imported from @tauri-apps/api/notification — Tauri v1 API, removed in v2	Updated to @tauri-apps/plugin-notification
2	Imported invoke from @tauri-apps/api/tauri — Tauri v1, removed in v2	Updated to @tauri-apps/api/core
3	Frontend called invoke('handle_notification') but no such Rust command existed	Added handle_notification Tauri command to notifications.rs, registered in lib.rs

Verification

• cargo check ✅ 0 errors 0 warnings
• dist/index.html ✅ Platform selector, no redirect
• dist/assets/main-*.js ✅ Bundled platform selector + notification JS
• Notification API imports ✅ All Tauri v2 compatible