fix(packaging): spawn Electron's node, relocate whisper, gitignore artifacts, drop unused dep

[thisiscsim]

Summary

Audit roadmap Phase 5 (shippability). Three things that work in dev but break a packaged build or dirty the repo:

• Engine scripts spawned via node from PATH with a repo-relative cwd — a packaged app has no system node, so every Generate/Export/Transcribe would fail with spawn node ENOENT. runScriptArgs now spawns process.execPath with ELECTRON_RUN_AS_NODE=1 (Electron's bundled Node), and generate-llm.mjs's nested spawnSyncs use process.execPath too (they inherit the flag).
• whisper.cpp + model installed into <repo>/.whisper — read-only in a packaged bundle, and ~140 MB one git add . from being committed. transcribe.mjs now honors APERTURE_WHISPER_DIR (main points it at userData/whisper); .whisper/ is gitignored as the CLI fallback, alongside storybook-static.
• Removed @remotion/transitions — declared in app/package.json but imported nowhere (verified across app/src, app/scripts, packages/); dropped from the lockfile.

Main-process + engine-script change — restart npm run dev.

Test plan

• node --check on the changed scripts; typecheck / lint / format / 101 tests / build green
• @remotion/transitions absent from package-lock.json, no source imports
• Manual (post-merge, dev restart): Generate/Transcribe still run (spawn via Electron node); whisper installs under userData/whisper

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