feat: choose which files to exclude before and during a download

[BRAINIFII]

What and why

Season packs and repacks often carry files you don't want — a sample clip, a second language track, bonus junk. Until now every file in a torrent came down; the only way to skip one was to let it finish and delete it afterward.

Add a file picker, reached with x on a search result. It fetches the torrent's file list (from the cached .torrent when present, otherwise a metadata-only swarm fetch that pulls no file data), then lets you drop files before the download starts. Single-file torrents and unreadable lists fall through to a normal download, so x never dead-ends.

Excluded files are applied by adding the torrent deselected and selecting only the kept files once metadata arrives, so skipped files never touch disk. The engine tracks the selected byte total and rescales progress/eta against it, since webtorrent's own progress counts the whole torrent and would otherwise stall below 100%. The choice rides on the QueueItem (and through history to seeds), so a resume, retry, or re-seed re-applies the same exclusions.

x in the Downloads pane re-opens the picker for a download already in flight, preloaded with its current selection. The change is applied to the live torrent in place — only the newly excluded files are deselected and the kept ones re-asserted, so selections never empty out (which would drop peer interest and stall the download). Already downloaded pieces stay on disk and are never re-fetched.

Wires the new x into both halves of the keymap (the ? sheet and the Downloads footer) and the requestFileSelection/requestReselect Store fields into makeStore, per CONTRIBUTING. Covered with vitest: engine select/deselect mapping and progress rescaling, queue persistence and live reselect, and the picker's toggle logic.

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.