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/web/src/lib/tauri.ts

@@ -0,0 +1,25 @@
+/**
+ * Lightweight Tauri environment detection and utilities.
+ * Avoids importing @tauri-apps/api at module level so the web build
+ * doesn't bundle Tauri internals.
+ */
+
+let _isTauri: boolean | null = null;
+
+export function isTauri(): boolean {
+  if (_isTauri !== null) return _isTauri;
+  _isTauri =
+    typeof window !== 'undefined' &&
+    '__TAURI_INTERNALS__' in window;
+  return _isTauri;
+}
+
+export async function openUrl(url: string): Promise<void> {
+  if (!isTauri()) {
+    window.location.href = url;
+    return;
+  }
+  // Lazy-import Tauri opener only in Tauri context
+  const { openUrl: tauriOpenUrl } = await import('@tauri-apps/plugin-opener');
+  await tauriOpenUrl(url);
+}