feat(desktop): manage bounded provider owner attestations

[dschwartzAI]

Outcome

Adds an official Buzz Desktop lifecycle for installing, rotating, and revoking bounded NIP-OA owner attestations on already-deployed provider agents.

Security properties

• Mints with Desktop's existing owner signing-key path; the owner private key never leaves Desktop.
• Permits only provider-backed managed agents with a deployed backend ID.
• Requires a 5-minute to 31-day lifetime and emits created_at<expiry.
• Sends the derived tag to the provider over stdin and redacts both the full tag and its signature from provider errors.
• Exposes only public metadata in the UI: state, owner pubkey, expiry, and a 12-character SHA-256 fingerprint.
• Supports rotation and revocation, with provider rollback if local persistence fails.
• Explicitly states that the credential does not grant channel roles, relay administration, deploy, database, Stripe, or migration authority.

Validation

• pnpm --dir desktop check
• pnpm --dir desktop typecheck
• pnpm --dir desktop test (3721/3721)
• cargo fmt --check
• cargo check
• cargo clippy --all-targets -- -D warnings
• focused Rust owner-attestation test
• production frontend build
• release Rust build with the system keyring feature

No application deployment is included in this PR.