feat(mobile): Android Digital Asset Links for passkey domain binding

[Osuolale1]

Closes #447

Summary

Set up the Android equivalent of passkey domain binding via Digital Asset Links, proving the app and website belong to the same entity. This is self-serve and needs no paid account, so it lands ahead of iOS and unblocks real-device passkey testing on Android.

• Added frontend/website/public/.well-known/assetlinks.json, served statically by the Next.js website at https://veil.app/.well-known/assetlinks.json. Declares com.veil.wallet as the linked Android app, with delegate_permission/common.get_login_creds (required for credential/passkey sharing) alongside delegate_permission/common.handle_all_urls.
• Added android.package: "com.veil.wallet" and an android.intentFilters entry with autoVerify: true for https://veil.app/* in frontend/mobile/app.json, matching the RP ID (veil.app) already used for passkeys elsewhere in the codebase (frontend/mobile/lib/passkey.ts, frontend/mobile/lib/__tests__/restore.test.ts).
• The sha256_cert_fingerprints value in assetlinks.json is a placeholder (REPLACE_WITH_APP_SIGNING_CERT_SHA256_FINGERPRINT) — needs the real signing-cert SHA-256 fingerprint (from the release keystore, or the Play App Signing console report) before the Digital Asset Links validator will pass.

Test plan

• Fill in the real signing-cert SHA-256 fingerprint in assetlinks.json
• Deploy the website so https://veil.app/.well-known/assetlinks.json resolves
• Run the Digital Asset Links API validator against veil.app / com.veil.wallet and confirm it passes
• On a real Android device/emulator, verify the passkey RP ID (veil.app) is accepted and a passkey created on web is usable from the app (or vice versa)

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[Miracle656]

Needs a small rebase. This edits frontend/mobile/app.json, but the mobile config moved to app.config.ts on main (app.json was removed). Please rebase and add the Android Digital Asset Links / intent-filter config to app.config.ts instead; the frontend/website/public/.well-known/assetlinks.json half looks fine. Then I'll re-review.

[Miracle656]

Thanks @Osuolale1 — reviewed. This is superseded by current main: PR #508 migrated the Expo config to app.config.ts, which already carries the Android App Links intentFilters (with autoVerify) and iOS associatedDomains, and the verification files already ship at frontend/wallet/public/.well-known/{assetlinks.json,apple-app-site-association}. This PR would also re-add frontend/mobile/app.json (which main deleted) and its assetlinks uses a different host (veil.app vs main's app.veil.xyz) and package (com.veil.wallet vs xyz.veil.wallet). Recommend closing — there's nothing to merge on top of current main. The domain-binding you set up is exactly what landed, so this counts.