feat(pwa): add install prompt, share target, and idle relock

[Hallab7]

Summary

Adds the missing PWA install, share-target intake, and app-level idle relock flows.

Changes

• Captures beforeinstallprompt and displays an Install action in the header.
• Permanently hides the install action after the native prompt is dismissed.
• Registers Wraith as a Web Share Target for shared text.
• Routes shared stellar: payment URIs to /send.
• Prefills the Stellar recipient, amount, and memo from shared URIs.
• Adds app-wide relocking after five minutes of inactivity.
• Clears in-memory stealth keys when the session locks.
• Supports passkey unlocking when available, with vault passphrase fallback.
• Extracts the existing KeyVault inactivity logic into a shared idle-lock utility.
• Handles mobile background timer throttling by checking elapsed time when the app becomes visible.

Testing

• Stellar URI parser tests pass: 6/6.
• Edited files pass Prettier checks.
• git diff --check passes.
• Edited TypeScript files transpile without syntax errors.
• Full build was blocked by missing/incomplete repository dependencies unrelated to this change.

closes #159

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