Improve mobile layout for dashboard hub navigation

## Context
Dashboard IA uses Talk / Knowledge / Inbox / Settings hubs. Sidebar may overflow on narrow viewports.

## Scope
- CSS-only responsive pass for `dashboard/static/styles.css`
- Collapsible sidebar or top nav under ~768px
- Playwright smoke optional (viewport 375px)

## References
- `dashboard/index.html`, `dashboard/static/app.js`

Start with dashboard/static/styles.css and inspect the navigation structure in dashboard/index.html, using dashboard/static/app.js for any existing navigation behavior. Test the dashboard at a 375px viewport, optionally with a Playwright smoke check. Done means the Talk, Knowledge, Inbox, and Settings navigation remains usable without sidebar overflow below about 768px.

Written by the indexing model from the issue text.