Wire desktop wallet coordination handlers

[kwsantiago]

Problem

The desktop wallet screen (keep-desktop/src/screen/wallet.rs) has complete UI/view layers for descriptor coordination — share selection, network picker, recovery tier configuration, progress display. But the message handlers are not wired into the main event loop, so the coordination flow doesn't actually work end-to-end.

Solution

Connect the wallet screen's coordination flow to KfpNode via the existing app message system.

What exists

• WalletScreen with SetupState configuration state machine
• DescriptorProgress enum tracking coordination phases
• view_configure() for policy editing
• view_coordinating() for progress display
• wallet_card() for listing completed descriptors
• derive_xpub() helper in frost.rs

What's missing

• Message handler in app.rs to dispatch WalletScreen messages to KfpNode
• KfpNodeEvent → Message mapping for descriptor events (Proposed, Contributed, Complete, Failed)
• Progress state updates as contributions/ACKs arrive
• Error handling and timeout display
• Store completed descriptor via keep.store_wallet_descriptor()

Dependencies

• Should include XpubAnnounce flow (Add XpubAnnounce pre-exchange for recovery keys #269) and key proof PSBT (Add key proof PSBT to DescriptorAck #270) for the full enhanced flow
• Follow the same handler wiring pattern used for FROST signing in the network screen