Add category image upload with thumbnail backdrop in category tiles

- Add category image API methods (upload/delete) to src/api/index.js
- Add uploadCategoryImage/deleteCategoryImage thunks and extraReducers in categorySlice
- Add CategoryBackdrop component showing ThumbHash placeholder with full thumb overlay
- Add photo upload/delete tab to CategoryEditTile settings cycle
- Category tile label band uses semi-transparent black overlay with white text
- Pass thumbHash/fullHash to both CategoryTile and CategoryEditTile

src/api/index.js

@@ -385,6 +385,27 @@ export const category = {
       body: { pos },
     });
   },
+
+  /**
+   * POST /api/category/:id/image
+   * Body: raw AVIF bytes (Content-Type: image/avif)
+   * → 201 { category: { id, fullHash, thumbW, thumbH, thumbHash } }
+   */
+  uploadImage(categoryId, file) {
+    return httpBinary(`/api/category/${categoryId}/image`, {
+      method: 'POST',
+      contentType: 'image/avif',
+      body: file,
+    });
+  },
+
+  /**
+   * DELETE /api/category/:id/image
+   * → 200 { category: { id, thumbW: null, thumbH: null, thumbHash: null } }
+   */
+  deleteImage(categoryId) {
+    return http(`/api/category/${categoryId}/image`, { method: 'DELETE' });
+  },
 };
 
 /**