fix: responsive header with hamburger menu for mobile

- Desktop: single row with logo, nav links, QuickAdd, New note button
- Mobile: logo + QuickAdd + hamburger icon → dropdown menu
- Improved QuickAdd sizing for small screens

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.claude/settings.local.json

@@ -9,7 +9,8 @@
       "Bash(node:*)",
       "Bash(curl:*)",
       "Bash(npx tsc:*)",
-      "Bash(npm list:*)"
+      "Bash(npm list:*)",
+      "Bash(npx jest:*)"
     ]
   }
 }