feat(viewer): progressive image preview via thumbnail

The photo viewer now renders the file's thumbnail (usually already
cached from grid view) immediately while the full entity URL
resolves, then swaps to the full image on load. Gives clients the
low-res-first preview path from the upstream request without a new
thumbnail-size pipeline; a dedicated preview-size entity variant
would be the follow-up if admins need larger previews.

Resolves #113

Authored By: TDvorak <info@tdvorak.dev>

frontend/src/component/Viewers/ImageViewer/react-photo-view/Photo.tsx

@@ -5,7 +5,9 @@ import React, { useEffect } from "react";
 import { getFileEntityUrl, getFileInfo } from "../../../../api/api.ts";
 import { EntityType, FileResponse, Metadata } from "../../../../api/explorer.ts";
 import { useAppDispatch } from "../../../../redux/hooks.ts";
+import { loadFileThumb } from "../../../../redux/thunks/file.ts";
 import { getFileLinkedUri } from "../../../../util";
+import { FileManagerIndex } from "../../../FileManager/FileManager.tsx";
 import { LRUCache } from "../../../../util/lru.ts";
 import FacebookCircularProgress from "../../../Common/CircularProgress.tsx";
 import useMountedRef from "./hooks/useMountedRef";
@@ -100,6 +102,18 @@ export default function Photo({
   };
 
   useEffect(() => {
+    // Progressive preview: show the (usually already-cached) low-res
+    // thumbnail immediately while the full entity URL resolves. (#113)
+    dispatch(loadFileThumb(FileManagerIndex.main, file))
+      .then((thumbUrl) => {
+        if (thumbUrl && mountedRef.current) {
+          setImageSrc(thumbUrl);
+        }
+      })
+      .catch(() => {
+        // No thumbnail available - the full entity load below still applies.
+      });
+
     dispatch(
       getFileEntityUrl({
         uris: [getFileLinkedUri(file)],