Add wallet descriptor coordination UI

[kwsantiago]

Integrate wallet descriptor coordination from keep-mobile UniFFI bindings into the Android app.

Upstream: privkeyio/keep#242

UniFFI API Surface

The keep-mobile bindings expose these new APIs (from keep_mobile.udl):

Types:

• WalletDescriptorInfo — group_pubkey, external/internal descriptor, network, created_at
• RecoveryTierConfig — threshold, timelock_months
• DescriptorProposal — session_id, network, tiers

KeepMobile methods:

• wallet_descriptor_list() → List<WalletDescriptorInfo>
• wallet_descriptor_export(group_pubkey, format) → String (formats: "sparrow", "raw")
• wallet_descriptor_delete(group_pubkey)
• wallet_descriptor_set_callbacks(callbacks)
• wallet_descriptor_propose(network, tiers) → session_id
• wallet_descriptor_cancel(session_id)
• wallet_descriptor_approve_contribution(session_id)

DescriptorCallbacks (implement in Kotlin):

• on_proposed(session_id) — proposal sent, waiting for contributions
• on_contribution_needed(proposal) — incoming proposal from another participant, show approval UI
• on_contributed(session_id, share_index) — a participant contributed their xpub
• on_complete(session_id, external_descriptor, internal_descriptor) — descriptor finalized
• on_failed(session_id, error) — session failed

All callback methods throw KeepMobileError (same pattern as BunkerCallbacks).

Android Work

1. DescriptorCallbacks implementation — Kotlin object : DescriptorCallbacks that bridges events to UI state (follow BunkerService/BunkerCallbacks pattern)
2. Wallet screen — list descriptors per group, export (Sparrow JSON / raw), delete
3. Propose flow — network picker, recovery tier config (threshold + timelock), calls wallet_descriptor_propose
4. Contribution approval — notification/dialog when on_contribution_needed fires, calls wallet_descriptor_approve_contribution
5. Session status — show progress (proposed → contributions → complete/failed) via callback state
6. Navigation — add Wallet route or integrate into existing Home/Share details screen