Support XpubAnnounce and key proof in WDC flow

[kwsantiago]

Problem

The wallet descriptor coordination UI (tracked in #164) will need to support two new protocol features being added to keep-mobile:

1. XpubAnnounce — participants broadcast their recovery xpubs (hardware wallet backup keys) before the initiator proposes a wallet policy
2. Key proof PSBT — when acknowledging a finalized descriptor, participants sign a dummy PSBT proving they control their contributed key

Without these, the Android app can only participate in the basic 4-message WDC flow but not the enhanced flow with authenticated recovery key exchange and key-control verification.

Solution

XpubAnnounce UI

• Add "Announce Recovery Keys" step before wallet proposal
• Let user select recovery xpubs from their device (hardware wallet, second device key)
• Call wallet_xpub_announce() via keep-mobile UniFFI (once exposed)
• Show announced keys from other participants before proposal starts

Key proof in ACK

• After receiving a finalized descriptor, automatically generate and sign the dummy PSBT
• Show user what they're signing ("Verifying key control for wallet descriptor")
• Include signed proof in the ACK sent back to initiator
• Call updated wallet_descriptor_approve_contribution() with proof

Dependencies

• Blocked on keep #269 (XpubAnnounce) and keep #270 (key proof PSBT) landing in keep-mobile
• Builds on Add wallet descriptor coordination UI #164 (wallet descriptor coordination UI)