feat: cherry-pick keyboard UX and file download from community PRs

Cherry-picked from PR #60 (keyboard UX) and PR #61 (file download):

- Alt+1-9 session switching
- Disable Ctrl+K (too easy to trigger accidentally)
- Session rename with prefix preservation (w1-case: description)
- Shift+Enter / Ctrl+Enter multiline input via tmux send-keys -H
- Android virtual keyboard fix for non-composition input
- File download button in browser file explorer (?download=true)

Dropped from PR #60: stale package-lock.json, upload popup (missing upload.html)
Dropped from PR #61: standalone /api/download endpoint (arbitrary fs access)
Fixed from PR #60: execFileSync replaced with async execFile

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>