Rebuild: export pipeline, render queue, Send to OpenPost

[rodrgds]

Part of the video editors rebuild. Spec: docs/specs/video-editors-rebuild.md

DONE (4a2ca5d):

• Single-clip export: mediabunny Conversion stream-copy remux with in/out trim, saved to projects/{id}/exports/ via workspace-fs exports module
• Send to OpenPost: uploadMediaFile with source=video_editor_export into the Media library
• Export/Send buttons in the editor tools panel

REMAINING:

• Multi-item timeline render: per-frame canvas renderer (CanvasSink decode -> canvas draw per project fps) + OfflineAudioContext mixdown -> CanvasSource/AudioSource -> Output encode; reference FreeCut features/export utils/canvas-render-orchestrator.ts + client-render-engine.ts (MIT), trimmed to v1 item types (video/audio/image/text/subtitle)
• Codec probing fallback chain (canEncodeVideo), WebM option in UI, render queue persistence to projects/{id}/render-queue.json
• Subtitle burn-in + .srt/.vtt sidecar export once Rebuild: transcription, text-based editing, silence removal, captions #123 lands

[rodrgds]

Multi-track rendered export landed in 5d85f4e2 (feat(video-editor): multi-track rendered export).

What landed

• frontend/src/lib/video-editor/media/render-export.ts — composed render of the whole timeline, ported from FreeCut (MIT) canvas-render-orchestrator: per-frame OffscreenCanvas compositing via mediabunny CanvasSink (contain-fit, transform/rotation/flip/opacity), crossfade/fade-black blending using the existing transition math, bottom-center subtitle cue burn-in, then VP9+Opus WebM encode via mediabunny Output/VideoSampleSource/AudioSampleSource with frame progress reporting. Saved through the existing exports folder flow.
• Audio mixdown: all audible video/audio items decoded to stereo and mixed once in a 48 kHz OfflineAudioContext — correct offsets, speed as playback rate, gain = item volume × track volume with keyframed-volume automation points; muted/solo track rules respected.
• frontend/src/lib/video-editor/media/render-plan.ts — pure planning math (output duration, frame→source-time mapping incl. speed/sourceFps, mixdown schedule, transition blend lookup, paint order, cue selection).
• UI: "Render full video" button next to Export MP4 in the editor tools panel with live frames-done/total progress bar; new i18n keys video_editor_export_render, video_editor_render_progress in en + pt.
• Tests: render-plan.test.ts — 22 vitest unit tests over timing mapping, mixdown offset/gain/automation, transitions, paint order, cue selection.

Verification: 101/101 vitest tests pass across src/lib/video-editor, svelte-check 0 errors, oxlint clean.

Not done / follow-ups: MP4/H.264 path is wired but untested against real codecs; text items are not yet composited into the render (preview doesn't draw them either); no cancel button for an in-flight render.