feat: implement mobile app usability improvements

Mobile Navigation:
- Add MobileNav component with bottom tab bar
- Show mobile nav on small screens, hide desktop sidebar
- Add session count badge to Sessions tab
- Add safe area padding for notched devices

Session Management:
- Redesign SessionCard for mobile with action menu
- Add MobileActionSheet for session actions
- Keep primary action prominent

Forms & Dialogs:
- Stack form fields vertically on mobile
- Ensure 44px minimum touch targets
- Update dialogs for 320px viewport

Responsive Layout:
- Add MobilePageHeader with back button
- Reduce page padding on mobile
- Stack multi-column grids vertically

Touch & Interaction:
- Add active states to interactive elements
- Ensure 8px spacing between touch targets

Complex Pages:
- Update Repo Workspace for mobile
- Update Tool Workshop and Config Profiles

Build: TypeScript check passes, production build succeeds

openspec/changes/mobile-app-usability/proposal.md

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+## Why
+
+The Headquarter web app is currently unusable on mobile devices. Critical workflows—starting and stopping sessions, viewing instance status, and making small configuration adjustments—are blocked by desktop-oriented layouts, missing touch targets, and broken responsive behavior. Since users perform real work through terminal sessions (already mobile-optimized) and session management, the core app shell and key pages must be at least functional on phones.
+
+## What Changes
+
+- **Mobile navigation**: Replace horizontal scroll nav with bottom tab bar on mobile breakpoints
+- **Session management mobile layout**: Redesign session cards for touch, add swipe actions, ensure all controls are reachable
+- **Dashboard mobile view**: Simplify summary cards, optimize quick actions for touch
+- **Responsive forms**: Stack multi-column forms vertically on mobile, ensure touch targets ≥ 44px
+- **Mobile-optimized dialogs**: Ensure all dialogs fit within 320px viewport, add scroll when needed
+- **Page headers**: Add consistent back buttons and contextual action bars on mobile
+- **Touch feedback**: Add active states and loading indicators for all interactive elements
+- **Graceful degradation**: Complex admin pages (Tool Workshop, Config Profiles) get read-first mobile views with edit capability
+
+## Capabilities
+
+### New Capabilities
+- `mobile-navigation`: Bottom tab bar, page transitions, and mobile-specific navigation patterns
+- `mobile-session-management`: Touch-optimized session cards, swipe actions, and mobile session controls
+- `mobile-responsive-layouts`: Breakpoint system, touch targets, and form stacking for mobile viewports
+
+### Modified Capabilities
+- `frontend-foundation`: Add mobile breakpoint system and responsive grid utilities
+- `session-lifecycle-ux`: Extend session management UX with mobile-specific interactions
+- `tool-instances`: Ensure instance controls (start/stop/restart) are usable on mobile
+
+## Impact
+
+- **Frontend**: Major CSS changes to AppShell, page layouts, form components, and card patterns
+- **Components**: New MobileNav component, updates to SessionCard, CreateSessionForm, and all page components
+- **User Experience**: Significantly improved mobile usability without reducing desktop functionality
+- **No API changes**: Purely frontend/CSS work