Make gallery page fully mobile responsive

GalleryLayout improvements:
- Stack header elements vertically on mobile
- Hide company branding on small screens
- Make dates stack vertically
- Responsive text sizes and padding
- Icon-only logout button on mobile
- Improved button layout with proper wrapping

PhotoFilterBar improvements:
- Stack search and sort vertically on mobile
- Full-width sort button on mobile
- Horizontally scrollable category filters
- Responsive text sizes
- Mobile-friendly dropdown positioning

PhotoGrid improvements:
- Responsive selection controls
- Touch-friendly photo overlays
- Larger selection checkboxes on mobile
- Improved button text for small screens
- Responsive gaps between photos

Gallery grid CSS:
- Smaller gaps on mobile devices
- Maintains 2 columns on smallest screens

GalleryPage login:
- Responsive padding and margins
- Smaller text and icon sizes on mobile
- Better card spacing
- Responsive form elements

UserPhotoUpload modal:
- Full-screen modal on mobile (slides up from bottom)
- Responsive padding and text sizes
- Mobile-optimized upload area
- Sticky footer on mobile

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

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

frontend/src/index.css

@@ -111,9 +111,9 @@
     @apply animate-pulse bg-neutral-200 rounded-lg;
   }
 
-  /* Gallery grid */
+  /* Gallery grid - Mobile optimized */
   .gallery-grid {
-    @apply grid gap-4 grid-cols-2 sm:grid-cols-3 md:grid-cols-4 lg:grid-cols-5 xl:grid-cols-6;
+    @apply grid gap-2 sm:gap-3 md:gap-4 grid-cols-2 sm:grid-cols-3 md:grid-cols-4 lg:grid-cols-5 xl:grid-cols-6;
   }
 
   /* Modal overlay */