feat(export): save clip/range as media (#48)

[appergb]

Summary

Wires up the Save as Media right-click action (#48 tail, HANDOFF §3.5): render a single clip or a marked timeline range to a file on disk and import it back into the media library, so the result appears in the panel and is undoable.

Changes

Backend (src-tauri/src/export.rs)

• New export_range command: renders a single clip (clip_id) or a marked range (in_frame..out_frame) of the whole-timeline composite.
• Extends run_export_with_control with an optional frame_range parameter (full-timeline export passes None → unchanged behavior); the frame loop, progress reporting, and audio mixdown are all sliced to the requested range.
• Reuses the SAME composite_frame / MediaResolver pixel path as the preview and full-timeline export → a saved clip drags back onto the timeline pixel-identical.
• Audio-only clips (media_type = audio) and explicit AudioWav format skip the GPU: mix down via opentake_media::encode::mix → hand-written 44-byte WAV header (no ffmpeg child for raw PCM).
• Output lands in <cache_root>/saves/save_<id>_<in>_<out>.{mp4,wav}, then import_one brings it into the manifest (same path as capture_frame_to_media).
• Progress streams via the existing export://progress event; cancel via the existing cancel_export flag.

IPC registration (src-tauri/src/lib.rs)

• export::export_range added to generate_handler!.

Frontend

• web/src/lib/types.ts: ExportFormat = "video" | "audioWav" (mirror of Rust enum, camelCase wire tag pinned by test).
• web/src/lib/api.ts: saveClipAsMedia(clipId, inFrame, outFrame, format?, trackIndex?) IPC wrapper; returns null outside Tauri.
• web/src/components/timeline/ClipContextMenu.tsx: "Save as Media" menu item (was a disabled placeholder) now wired — resolves clip span + track index, picks audioWav for audio clips, subscribes to progress, toasts success/failure.
• web/src/i18n/dict.ts: media.saveAsMediaSaved / media.saveAsMediaFailed toast copy (zh + en).

Acceptance

• Video clip with color grade + text → Save as Media → re-dragged clip looks identical (same RenderPlan)
• Audio-only clip → exports .wav (auto-downgrade Video→AudioWav)
• Marked range (clip_id=null) → whole-timeline composite for that span
• Output appears in media panel + undoable (via import_one manifest mutation)

Local verification (all green)

cargo fmt --all --check                                  ✓
cargo clippy --workspace --all-targets -- -D warnings    ✓
cargo clippy -p opentake-tauri --no-default-features --all-targets -- -D warnings  ✓
cargo test --workspace                                   ✓ (36 export tests, incl. 7 new)
pnpm -C web build                                        ✓
pnpm -C web test                                         ✓ (437 passed)

New unit tests: export_format_defaults_to_video, export_format_parses_audio_wav_tag (IPC camelCase pin), slice_pcm_* (3), write_wav_s16le_writes_valid_riff_header.

Closes #48.