fix(main): async project listing, ffmpeg timeouts + concurrency cap, structured IPC errors, recorder cleanup

[thisiscsim]

Summary

Audit roadmap Phase 5 (main-process robustness + two renderer leak/error fixes).

Main process

• listProjects is now async (fs/promises + Promise.all). It parses every project's edl.json + meta.json; doing that serially with sync fs on the main (UI) thread janked the window once a user had many projects.
• Shared runFfmpeg(args, timeoutMs) with a hard SIGKILL timeout replaces the four ad-hoc ffmpeg spawns (thumbnail / duration probe / proxy / voice-sample). A malformed or hostile media file could otherwise hang a spawn forever with no reaping. Proxy encodes are bounded to 2 concurrent — importing N videos previously fired N ffmpeg processes at once, saturating CPU/memory.
• Structured IPC errors: asset:import, asset:importBuffer, references/benchmark:import, project:watch, and project:thumbnail validate the slug via a shared assertSlug and return { ok:false, error } instead of throwing across IPC (they called safeProjectPath, which throws, outside any try/catch — violating the repo's own IPC convention). asset:importBuffer also rejects a non-basename filename.
• meta:save / style:patch re-validate the merged result through parseMeta / parseStyleProfile before writing, so an arbitrary renderer patch can't persist unknown keys or an unsafe (url()) palette into the sidecar files.

Renderer

• RecordButton (LeftRail) and VoicesTab (Settings) now stop the MediaRecorder and release the mic tracks on unmount. Cmd+\ unmounts the rail mid-recording, which left the mic hot and the recorder leaked.
• Home project/album delete failures now surface an error toast instead of silently doing nothing.

Main-process + preload-adjacent change — restart npm run dev.

Test plan

• typecheck / lint / format / 101 tests / build green
• Manual (post-merge): import several videos → at most 2 ffmpeg proxies run at once; a bad slug from a hacked renderer returns an error (no unhandled rejection); start a mic recording then Cmd+\ → mic light turns off; delete a project while the folder is locked → error toast

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