Add mobile save sheet with same-origin photo downloads.

Phones cannot land downloads in the photo library, so the gallery opens a share sheet for a cacheable preview while streaming via a dedicated download endpoint and recording preview sizes.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/tailwind.config.js

@@ -37,6 +37,18 @@ module.exports = {
         'card': '0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px rgba(0, 0, 0, 0.06)',
         'card-hover': '0 10px 15px -3px rgba(0, 0, 0, 0.1), 0 4px 6px -2px rgba(0, 0, 0, 0.05)',
       },
+      // Entrance for the mobile save sheet (components/gallery/SaveSheet).
+      keyframes: {
+        'sheet-fade': { from: { opacity: '0' }, to: { opacity: '1' } },
+        'sheet-rise': {
+          from: { transform: 'translateY(100%)' },
+          to: { transform: 'translateY(0)' },
+        },
+      },
+      animation: {
+        'sheet-fade': 'sheet-fade 150ms ease-out',
+        'sheet-rise': 'sheet-rise 200ms ease-out',
+      },
     },
   },
   plugins: [],