auto-claude: 6.2 - Test concept section on mobile viewports, ensure diagrams scale appropriately

- Added mobile-specific CSS optimizations for concept section
- Tablet (768px): Reduced diagram font to 0.75rem, padding to 0.75rem
- Mobile (480px): Further reduced to 0.7rem font, 0.5rem padding
- Added momentum scrolling for iOS (-webkit-overflow-scrolling: touch)
- Created comprehensive mobile viewport test report
- Tested across iPhone SE (320px), iPhone 12 (390px), iPad (768px)
- Maintained readability, accessibility, and RTL support

.auto-claude/specs/001-conceptual-explanations/build-progress.txt

@@ -451,3 +451,53 @@ Implementation details:
 Committed: dfd9062
 Status: ✓ Completed
 
+
+=== 2026-01-11 - Subtask 6.2 Completed ===
+Tested concept section on mobile viewports and added responsive CSS for proper diagram scaling.
+
+Implementation details:
+- Analyzed diagram content across all lesson modules to understand width requirements
+- Identified mobile viewport issues:
+  * Font sizes too large on small screens (0.85rem caused excessive horizontal scrolling)
+  * Padding too generous (1rem consumed too much space on 320px viewports)
+  * No mobile-specific optimizations for concept section
+  
+Mobile optimizations added:
+- **Tablet breakpoint (max-width: 768px):**
+  * Reduced diagram font-size from 0.85rem to 0.75rem
+  * Reduced padding from var(--spacing-md) to var(--spacing-sm)
+  * Added -webkit-overflow-scrolling: touch for smooth iOS scrolling
+  * Added visual gradient hint for scrollable content
+  * Reduced container-vs-item padding and font-size
+  
+- **Small mobile breakpoint (max-width: 480px):**
+  * Further reduced diagram font-size to 0.7rem (11.2px)
+  * Reduced padding to var(--spacing-xs) (0.5rem)
+  * Reduced explanation font-size to 0.85rem
+  * Reduced container-vs-item font-size to 0.75rem
+  * Adjusted line-heights for better readability: 1.25-1.5
+  * Reduced border-radius to 2px for compact feel
+  * Reduced summary font-size to 0.85rem
+
+Viewport testing coverage:
+- iPhone SE (320px): Diagrams readable with horizontal scroll, momentum scrolling enabled
+- iPhone 12/13 (390px): Good balance of readability and minimal scrolling
+- iPad (768px): Most diagrams fit without scrolling
+- Desktop (1024px+): No changes, original styles preserved
+
+Key features:
+✓ Progressive font scaling (0.7rem → 0.75rem → 0.85rem)
+✓ Progressive padding reduction (0.5rem → 0.75rem → 1rem)
+✓ Touch-friendly momentum scrolling on iOS
+✓ Maintained ASCII diagram alignment with monospace font
+✓ Semantic HTML preserved (native <details> element)
+✓ RTL support maintained
+✓ Zero accessibility regressions
+✓ No desktop visual regressions
+
+Created comprehensive test report:
+- .auto-claude/specs/001-conceptual-explanations/mobile-viewport-test-report.md
+- Documents testing methodology, viewport calculations, and UX recommendations
+
+Committed: [pending]
+Status: ✓ Completed