feat: add post-capture guidance with file location and verification hint

[felippemsc]

Summary

• Show "Saved to Files → SignedShot" after successful capture so users know where to find their files
• Add CLI verification hint ($ signedshot verify <photo> <sidecar>)
• Add "Open in Files" button to jump directly to the Files app
• Hide "Start Session" card when captured photo is displayed to reduce clutter

Test plan

• Capture a photo and verify the post-capture card shows file location, CLI hint, and "Open in Files" button
• Verify "Start Session" card is hidden while the captured photo preview is showing
• Tap "Open in Files" and confirm it opens the Files app
• Tap "Dismiss" and confirm the session prompt reappears