Desktop: iPhone HEIC photos upload as a single 512×512 tile (-map 0:v:0 selects the first grid tile) #7666

Describe the bug
Attaching a HEIC photo taken with an iPhone camera in Buzz Desktop uploads only one
512×512 tile of the image instead of the full photo. No error is shown.

Steps to reproduce

1. Take a photo on an iPhone with the default "High Efficiency" format (HEIC, 4032×3024).
2. Copy the original file to the computer.
3. Attach it to a message in Buzz Desktop and send.
4. The message shows a 512×512 cutout of the photo.

Expected behavior
The full photo is uploaded (3024×4032 after rotation).

Version and platform

• Buzz version: 0.5.23
• OS: Windows 11 Pro (ffmpeg 9.0.1, gyan.dev full build via winget)

Logs / additional context
iPhone HEICs are tiled HEIF images: ffprobe -show_stream_groups shows a "Tile Grid"
of 48 × 512×512 HEVC tiles. transcode_heic_to_jpeg passes -map 0:v:0, which
selects only the first tile:

Reproducible with plain ffmpeg:

# as in Buzz Desktop -> 512x512
ffmpeg -i IMG.HEIC -map 0:v:0 -map_metadata -1 -frames:v 1 -q:v 2 out.jpg
# without -map -> 3024x4032 (ffmpeg inserts xstack for the tile grid)
ffmpeg -i IMG.HEIC -map_metadata -1 -frames:v 1 -q:v 2 out.jpg

Possible fix: drop -map 0:v:0. Automatic tile-grid assembly in the ffmpeg CLI only
exists since FFmpeg 8.1, so older versions need a version check, an explicit xstack
filtergraph, or a hard error when the output size doesn't match the grid — instead of
silently uploading one tile. The Live Photo / burst case from the doc comment should
be re-checked. No sample attached (EXIF metadata); any iPhone camera HEIC reproduces it.