feat(web): mobile image upload to active session via paste dialog (#101)

- Extend the keyboard accessory paste dialog with an image picker
  (camera / photo library) plus best-effort image paste, routing
  selected files through the existing _uploadAndInsertImages pipeline
- Re-encode images to standard JPEG/PNG in the browser before upload,
  so mislabeled gallery images (e.g. MIUI WebP claiming image/jpeg)
  pass the server magic-byte check; PNG keeps transparency, GIF passes
  through untouched, decode failures fall back to the original file
- Log the real byte header on the paste-image magic-mismatch branch to
  pin down any remaining format mismatches without a guessing loop
- Ignore the runtime .claude-images/ upload directory

Co-authored-by: Teigen <teigen@TeigendeMac-mini.local>