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/src/lib/api/photos.ts

@@ -41,6 +41,8 @@ export interface Photo {
   originalFilename?: string;
   contentType: string;
   sizeBytes: number;
+  /** Size of the preview variant; absent until the server has measured it. */
+  previewSizeBytes?: number;
   width?: number;
   height?: number;
   takenAt?: string;
@@ -51,6 +53,15 @@ export interface Photo {
     thumb?: string;
     preview?: string;
     original: string;
+    /**
+     * Download endpoints. Unlike `preview`/`original` these always stream
+     * same-origin instead of redirecting to S3, so fetch() can read them
+     * into a Blob (needed by the mobile share sheet) without CORS.
+     * `download` serves the preview and is what the lightbox renders, so
+     * saving it hits the HTTP cache instead of the network.
+     */
+    download?: string;
+    downloadOriginal: string;
   };
   // Upload responses only: these bytes were already in the gallery, so nothing
   // was stored and the rest of this object describes the existing photo.