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/specs/frontend-foundation/spec.md

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+## MODIFIED Requirements
+
+### Requirement: Layout Component
+The system SHALL provide a consistent application layout for authenticated screens across desktop and mobile sizes.
+
+#### Scenario: Application shell
+- **GIVEN** the frontend application
+- **THEN** a Layout component SHALL:
+  - Display a header with user info and logout
+  - Display sidebar navigation on desktop (width >= 768px)
+  - Show main content area
+  - Collapse sidebar into a mobile menu toggle on small viewports
+  - **MODIFIED**: Display a bottom tab bar on mobile (width < 768px) with tabs for Home, Projects, Sessions, Tools, Settings
+  - **MODIFIED**: Hide sidebar navigation on mobile viewports
+
+#### Scenario: Navigation links
+- **GIVEN** the sidebar navigation
+- **THEN** it SHALL include links to:
+  - Dashboard
+  - Projects
+  - Repositories
+  - SSH Keys
+  - Settings
+- **MODIFIED**: The bottom tab bar SHALL include tabs for:
+  - Home (Dashboard)
+  - Projects
+  - Sessions
+  - Tools (Tool Workshop)
+  - Settings
+
+### Requirement: Responsive Design
+The system SHALL support mobile devices.
+
+#### Scenario: Mobile viewport
+- **GIVEN** a mobile device
+- **WHEN** the app loads
+- **THEN**:
+  - **MODIFIED**: A bottom tab bar is displayed for primary navigation
+  - Content adapts to screen width
+  - Touch targets are appropriately sized (minimum 44px)
+  - **ADDED**: All multi-column layouts stack vertically
+  - **ADDED**: Dialogs fit within the viewport and are scrollable
+  - **ADDED**: Page headers include back buttons where applicable