fix: keep canvas media local and improve load performance - #8

Summary

• keep canvas cards backed by durable local asset URLs while provider uploads remain generation-scoped
• persist generated images, videos, and video covers into project-owned local storage
• migrate legacy provider-hosted snapshot media with pnpm media:localize
• render only visible React Flow nodes to remove initial canvas media pressure
• update local-first documentation and regression coverage

Verification

• pnpm test: 256 passed
• pnpm typecheck: passed
• pnpm i18n:check: passed
• pnpm build: passed
• pre-commit security scan: clean
• browser refresh: zero remote BeatAPI media requests and zero Loading preview placeholders after one second

Local data boundary

The ignored data directory, SQLite database, and project media files are not included in this PR.