feat(android): Google OAuth via system browser + deep link for Android

Background: Google blocks OAuth in embedded WebView (403 disallowed_useragent).
Solution: open Google login in the Android system browser, then deep-link
back to the Tauri app via custom scheme after callback.

Changes:
- Tauri: add tauri-plugin-opener + tauri-plugin-deep-link to Cargo.toml
- Tauri: register plugins in lib.rs, add capabilities
- Web: auth-form.tsx Google button uses openUrl() via @tauri-apps/plugin-opener
  on Tauri (opens in system browser), falls back to window.location.href
- Web: add lib/tauri.ts for isTauri() detection + lazy opens
- API: /auth/google accepts ?platform=tauri → encodes into OAuth state param
- API: /auth/google/callback decodes state → if tauri, renders HTML page
  that deep-links back via zeavisedu:// scheme; if web, 302 redirect
- Android: patch script adds deep link intent filter for zeavisedu:// scheme

Co-Authored-By: Claude <noreply@anthropic.com>

apps/tauri/capabilities/default.json

@@ -3,6 +3,9 @@
   "description": "Capability for the main window",
   "windows": ["main"],
   "permissions": [
-    "core:default"
+    "core:default",
+    "opener:default",
+    "opener:allow-open-url",
+    "deep-link:default"
   ]
 }