feat: Complete high-priority frontend features - accessibility and UX

**Error Boundaries (VERIFIED COMPLETE)**
- Comprehensive ErrorBoundary component already implemented in components/common/ErrorBoundary.tsx
- Features: Recovery UI, error logging, isolated error boundaries, development error details
- Global error boundary in root layout
- Page-level error boundaries in AI Assistant and dashboard
- Error tracking integration ready for Sentry

**Touch Target Sizes (WCAG 2.5.5 Compliance)**
- Fixed user menu IconButton: increased from 32x32px to 44x44px minimum (medium size)
- Created lib/utils/touchTargets.ts with accessibility utilities:
  * MINIMUM_TOUCH_TARGET = 44px (iOS/WCAG standard)
  * RECOMMENDED_TOUCH_TARGET = 48px (Android Material Design)
  * Helper functions: withTouchTarget(), validateTouchTarget()
  * Component-specific guidelines for IconButton, Button, FAB, Chip, etc.
- Verified existing components meet standards:
  * Quick action buttons: 140x140px ✓
  * Bottom navigation: 64px height ✓
  * Voice FAB: 56x56px ✓
  * Voice button in tab bar: 48x48px ✓

**AI Conversation History (VERIFIED COMPLETE)**
- Comprehensive conversation management already implemented in AIChatInterface.tsx
- Features verified:
  * Full conversation list with drawer (mobile) and sidebar (desktop)
  * Load conversations from backend API
  * Load individual conversation messages with scrolling
  * Auto-scroll to bottom on new messages
  * Conversation groups with collapsible organization
  * Delete conversations with confirmation
  * Context menu for conversation management
  * Thinking messages animation
  * Markdown rendering for AI responses
- Updated implementation-gaps.md to reflect completion status

**Documentation Updates**
- Updated docs/implementation-gaps.md:
  * Marked Conversation History as COMPLETED
  * Updated AI Assistant UI section with detailed implementation notes
  * Moved remaining features (Streaming Responses, Suggested Follow-Ups, AI Response Feedback UI) to "Remaining Features" section

**Impact**
- Error boundaries prevent full app crashes and provide graceful recovery
- Touch target sizes meet WCAG 2.5.5 (AAA) and mobile platform guidelines
- Conversation history enables contextual AI interactions with full persistence

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

maternal-web/components/layouts/AppShell/AppShell.tsx

@@ -120,16 +120,20 @@ export const AppShell = ({ children }: AppShellProps) => {
         {/* User Menu Button - Top Right */}
         <IconButton
           onClick={handleMenuOpen}
-          size="small"
+          size="medium"
           aria-label="user menu"
           aria-controls={anchorEl ? 'user-menu' : undefined}
           aria-haspopup="true"
           aria-expanded={anchorEl ? 'true' : undefined}
+          sx={{
+            minWidth: 44,
+            minHeight: 44,
+          }}
         >
           <Avatar
             sx={{
-              width: 32,
-              height: 32,
+              width: 36,
+              height: 36,
               bgcolor: 'primary.main',
               fontSize: '0.875rem',
             }}