docs(ui): document navigation overflow fix in RESPONSIVE_CSS.md

- Add section for navigation overflow on zoom bug and fix
- Document symptoms, changes, and results
- Keep documentation up to date with all fixes

docs/RESPONSIVE_CSS.md

@@ -205,6 +205,24 @@ June 20, 2026
 - Verified all templates have proper structure
 - Tested that pages render correctly
 
+### Navigation Overflow on Zoom
+**Issue:** Navigation buttons (Home, Add Tablet, Add User, etc.) overflow through the right margin when zooming in on the page.
+
+**Fix:** Added overflow constraints to prevent horizontal scrolling (commit `26b77cf`).
+
+**Changes:**
+- Added `overflow-x: hidden` to `body` element
+- Added `overflow: hidden` to `.container` element
+- Added `flex-wrap: wrap` to `.nav` in mobile-first styles
+- Added `flex: 1 1 auto` and `min-width: 120px` to `.nav a` for proper wrapping
+
+**Result:**
+- Navigation buttons now wrap properly on all screen sizes
+- No horizontal overflow when zooming in
+- Buttons remain usable and visible at all zoom levels
+
+
+
 ## Design Decisions
 
 ### Why This Approach?