app: deep linking — universal links (iOS) + app links (Android)

Context

Sub-issue of #535. Depends on #649. Enables URLs like https://app.need4deed.org/dashboard/volunteers/42 to open the native app directly at that screen — used by push notifications and external shares.

iOS — Universal Links

1. Host /.well-known/apple-app-site-association on app.need4deed.org:
   ```json
   {
   "applinks": {
   "apps": [],
   "details": [{ "appID": "TEAMID.org.need4deed.app", "paths": ["/dashboard/", "/forms/"] }]
   }
   }
   ```
2. Enable Associated Domains in Xcode → Signing & Capabilities: applinks:app.need4deed.org
3. Handle the incoming URL in Capacitor:
   ```ts
   import { App } from '@capacitor/app';
   App.addListener('appUrlOpen', ({ url }) => {
   const path = new URL(url).pathname;
   router.push(path);
   });
   ```

Android — App Links

1. Host /.well-known/assetlinks.json on app.need4deed.org:
   ```json
   [{
   "relation": ["delegate_permission/common.handle_all_urls"],
   "target": { "namespace": "android_app", "package_name": "org.need4deed.app", "sha256_cert_fingerprints": ["..."] }
   }]
   ```
2. Add intent filter to android/app/src/main/AndroidManifest.xml for https://app.need4deed.org
3. Same appUrlOpen listener handles Android too

Hosting the well-known files

Add both files as static assets in public/.well-known/ — Next.js serves public/ at the root automatically.

Tasks

• Create public/.well-known/apple-app-site-association
• Create public/.well-known/assetlinks.json
• Enable Associated Domains in Xcode
• Add intent filter in AndroidManifest.xml
• Install @capacitor/app and wire appUrlOpen → router.push
• Test: tap a need4deed.org link on device → app opens at correct screen

Depends on

#649 (Capacitor setup), #651 (push notifications — deep links used there)