Improve error handling and recovery UX for capture flow

[felippemsc]

Summary

• Add live countdown timer for capture sessions (updates every second)
• Show "Session Expired" UI with orange "Create New Session" button when session expires
• Add "Retry" button on error alerts for network failures (register, start session, capture)
• Auto-recover from 401 unauthorized by clearing stale credentials and prompting re-registration
• Rewrite all user-facing error messages to be friendlier and actionable (no HTTP status codes, no technical jargon)

Test plan

• Start session and watch countdown tick down live
• Let session expire — verify "Session Expired" UI appears with "Create New Session" button
• Toggle airplane mode and attempt an action — verify "Retry" and "Dismiss" buttons appear
• Tap "Retry" after restoring network — verify action succeeds
• Simulate 401 (reset device on server) — verify app clears credentials and shows registration prompt
• Verify all error messages are user-friendly (no raw status codes or technical details)