feat(media): transcode HEIC/HEIF to JPEG on desktop upload

[tellaho]

Category: fix

User Impact: iPhone HEIC/HEIF photos now preview and upload correctly in the desktop chat composer instead of showing a blank preview and shipping an unviewable file.

Problem: HEIC/HEIF (Apple's default iPhone photo format) didn't render in the desktop chat composer — the preview went blank and the uploaded file was unviewable for everyone. Shend and tho both hit it; tho had to manually convert to JPEG. Slack handles HEIC fine, so this was a visible gap. Mobile already transcodes HEIC→JPEG before upload; desktop did not.

Solution: Transcode HEIC/HEIF to JPEG in Rust before upload, mirroring the existing video-transcode branch. Because the composer previews the server-returned blob URL of the uploaded bytes (not a local File), a single upload-side fix covers both upload and preview — no JS decode lib or canvas pass needed. Detection mirrors mobile's full ftyp brand set so the two platforms stay consistent.

File changes

desktop/src-tauri/src/commands/media_transcode.rs
Adds `is_heic_file(buf)` (ftyp box at offset 4 + mobile's full brand set `{heic,heix,hevc,hevx,heim,heis,mif1,msf1}`), `has_heic_extension(path)` for the picker path, and `transcode_heic_to_jpeg` (`ffmpeg -y -loglevel error -i -frames:v 1 -q:v 2 <out.jpg>`, 60s timeout, same anti-deadlock pattern as the video branch). 11 unit tests covering the brand table (including variants the `infer` crate misses), JPEG-not-HEIC, empty input, plus an ffmpeg-gated round-trip.

desktop/src-tauri/src/commands/media.rs
Wires HEIC detection + transcode into both upload paths (`process_picked_path` and `upload_media_bytes`) ahead of the existing video/passthrough branch. Resulting JPEG is re-detected by `detect_and_validate_mime`. Missing ffmpeg is a hard error, consistent with the video path — no silent fallback.

Reproduction Steps

1. Run the desktop app.
2. In the chat composer, paste or pick an iPhone HEIC photo (both clipboard and file-picker paths are covered).
3. The preview should render correctly, and the uploaded image should be viewable by everyone (it ships as JPEG).

Notes

• Detection uses magic bytes (ftyp brands) as the source of truth on the paste/drag byte path; the picker path also checks the file extension.
• ffmpeg is required for HEIC, same as for video.