docs: add mobile responsiveness guidelines to contributing

Author: xcutiboo

CONTRIBUTING.md

@@ -89,6 +89,16 @@ This is an Angular 21 project using standalone components and signals.
 - **Color palette** — use `kitty-*` colors from `tailwind.config.js`
 - **Branding** — pink (`pink-*`) colors reserved for Confitty accents
 
+### Mobile Responsiveness
+
+- **Mobile-first** — base styles for mobile, use `lg:` and `sm:` for larger screens
+- **Touch targets** — buttons should be at least `w-9 h-9` (36px) for easy tapping
+- **Responsive text** — use `text-xs lg:text-sm` for readable scaling
+- **Spacing** — use `p-4 lg:p-8` and similar responsive spacing
+- **Hide on mobile** — use `hidden lg:block`, `hidden sm:inline` as needed
+- **Sidebar** — collapsible on mobile with overlay backdrop
+- **Preview panel** — full-screen overlay on mobile, side panel on desktop
+
 ### Naming
 
 | Type | Convention | Example |