feat: improve mobile orchestrator worktree filtering UX

[web3dev1337]

Summary

• Added a mobile header worktree sidebar toggle button so the worktree list and filter controls are reachable from small screens.
• Synced sidebar toggle state/ARIA on both desktop and mobile toggles.
• Fixed mobile worktree selection behavior so the sidebar no longer auto-closes after each visibility toggle, allowing rapid multi-select interactions.

Files changed

• client/index.html
• client/styles.css
• client/app.js

Notes

• No server-side changes.
• This PR is UI-only and preserves the existing desktop sidebar behavior.