Add per-phase timeouts to descriptor coordination

[kwsantiago]

Descriptor sessions currently have a single 10-minute timeout (DESCRIPTOR_SESSION_TIMEOUT_SECS = 600). This makes it hard to distinguish between a stuck contribution phase vs a stuck finalize phase.

Add per-phase timeouts so each phase has an appropriate deadline:

• Contribution phase: 5 minutes from proposal (waiting for xpubs)
• Finalize phase: 2 minutes from all contributions received (waiting for initiator to assemble)
• ACK phase: 2 minutes from finalize sent (waiting for peer verification)

This enables:

• More precise timeout events (DescriptorFailed with phase-specific reason)
• Faster retry on contribution stalls without waiting the full 10 minutes
• Better UX — the desktop/mobile UI can show which phase is stuck

Changes needed:

• Add phase timestamps to DescriptorSession (e.g. contributions_complete_at: Option<Instant>, finalized_at: Option<Instant>)
• Add per-phase timeout constants to protocol.rs
• Check phase-specific timeouts in is_expired() based on current state
• Emit phase-specific timeout reasons in cleanup