Fix Linq wallet authorization formatting and connection flow

[fmhall]

Linq flattened the wallet authorization prompt into wallet.Connect and concatenated its connection URL. Send authorization instructions and URLs as native Linq text/link parts, preserving whitespace and using an attempt-specific idempotency key.

Wallet connection links now open an authenticated /api/link redirect to Link consent, skipping the wallet landing page and extra Connect click. The route checks challenge ownership and expiry before starting the existing Better Auth OAuth flow. Purchase approval URLs remain direct Link URLs, preserved verbatim and sent as native link previews.

Validation:

• pnpm check — all six tasks passed; 777 tests across 88 files.
• pnpm build — passed.
• pnpm exec eve build — passed.
• Focused regression coverage verifies native text/link delivery, replay idempotency, unchanged approval URL query parameters, and the OAuth lifecycle including ownership, expiry, encrypted tokens, refresh, and disconnection.
• Independent read-only review found no issues. Live Link/Linq delivery was not exercised.