fix: isolate downloads, seeding, and completed subfolders with legacy…

[kaiserc]

… file migration and tests

What and why

Checklist

• npm run typecheck is clean
• npm test passes
• New logic has a test (vitest; mock node built-ins for platform code)
• If I added a key, I updated both HELP_GROUPS and footerHints in src/ui/keymap.ts
• If I added a Store field, I updated makeStore in scripts/render-previews-impl.tsx
• OS-touching code works on Windows, macOS, and Linux
• One concern, with a Conventional Commits title (feat: / fix: / docs: / chore:)

New here? CONTRIBUTING.md explains each of these with examples from real merged PRs.