# Fix Caption Export and Add Burn-In Option

## Task
Exported clips were not including captions despite the "include captions" option being checked. Additionally, the user requested a "burn-in" option to render subtitles directly into the video frames.

## Root Causes

1. **Precise mode burn-in (timestamp offset)**: `-ss` was placed before `-i` (input seeking), shifting output PTS to 0. The `subtitles` filter reads the original VTT with absolute timestamps (e.g., cues at 300s), but the output video starts at 0s — no cues matched.

2. **Lossless mode muxing (VTT formatting)**: YouTube auto-generated VTT has karaoke-style `<c>` tags, inline timestamps (`<00:00:00.599>`), and positioning metadata (`align:start position:0%`) that confuse ffmpeg's VTT parser when converting to `mov_text`.

3. **Silent fallback**: When the ffmpeg subtitle export failed, the code silently fell back to exporting without subtitles, making the failure invisible to the user.

## Changes Made

### `src-tauri/src/services/clip_exporter.rs` (major rewrite)
- **Added `sanitize_vtt_for_ffmpeg()`**: Strips karaoke `<c>` tags, inline timestamps, and positioning metadata from VTT files. Writes a cleaned temp file.
- **Added `strip_vtt_tags()`**: Helper to remove all HTML-like tags from VTT cue text lines.
- **Replaced `build_ffmpeg_args_with_subs()`** with two separate functions:
  - `build_ffmpeg_args_mux_subs()`: Muxes subtitles as a track. Uses output seeking (`-ss`/`-to` after `-i`) for correct timestamp alignment. Works with both lossless and precise cut modes.
  - `build_ffmpeg_args_burnin_subs()`: Burns subtitles into video using the `subtitles` filter. Uses output seeking so the filter reads correct VTT timestamps. Always re-encodes (overrides to H.264/AAC if lossless mode selected).
- **Updated `export_single_clip_with_subs()`**: Now takes `burn_in: bool`, sanitizes VTT before use, dispatches to mux or burn-in builder. **Removed the silent fallback** — errors now surface to the user.
- **Updated `export_merged_with_subs()`**: Takes `burn_in: bool`, passes through to per-segment export.
- **Updated tests**: Replaced old tests for removed function, added tests for both new builders, added a VTT tag stripping test. All 40 tests pass.

### `src-tauri/src/models.rs`
- Added `pub burn_in_captions: bool` to `ExportConfig`.

### `src-tauri/src/commands/export.rs`
- Threads `config.burn_in_captions` through to `export_single_clip_with_subs` and `export_merged_with_subs`.

### `src/lib/bindings/export.ts`
- Added `burnInCaptions: boolean` to the TypeScript `ExportConfig` interface.

### `src/lib/components/ExportDialog.svelte`
- Added `burnInCaptions` state.
- Added "Burn into video" sub-checkbox under the "Include captions" checkbox.
- Contextual notes: "Captions will be muxed as a subtitle track" vs "Captions will be burned into the video" vs "Burn-in requires re-encoding (precise mode will be used)".
- Threads `burnInCaptions` through to the export config.

## Lessons Learned
- ffmpeg's `-ss` before `-i` (input seeking) adjusts output PTS to start at 0, but the `subtitles` filter reads timestamps from the original VTT file — they must match. Output seeking (`-ss` after `-i`) preserves original timestamps.
- YouTube auto-generated VTT files contain karaoke formatting that ffmpeg's VTT-to-mov_text converter can't handle cleanly. Sanitizing before use is essential.
- Silent fallbacks that hide errors waste debugging time. Surface errors to the user.

## Follow-up
- The CORS fix from the prior session (media_server.rs adding `CorsLayer::permissive()`) is also needed for captions to display in the preview player.