feat(ui): limit visible reactions and overhaul retro theme

- Limit the number of initially visible reactions per file to 2 and calculate the overflow count on the backend.
- Redesign the retro theme CSS to mimic a classic Windows 98 Explorer window, including title bars, toolbars, and sunken panes.
- Add local storage persistence for the file browser view preference (list vs. thumbnails).

backend/static/css/90-responsive.css

@@ -95,6 +95,41 @@
     flex: 1;
   }
 
+  .file-browser-toolbar {
+    align-items: stretch;
+  }
+
+  .file-browser-toolbar,
+  .file-browser-toolbar .view-toolbar {
+    width: 100%;
+  }
+
+  .file-browser-toolbar .view-toolbar .button,
+  .file-browser-toolbar > .button {
+    flex: 1 1 auto;
+    justify-content: center;
+  }
+
+  .file-browser-toolbar .view-toolbar .icon-button {
+    flex: 0 0 2.5rem;
+  }
+
+  .file-browser-head {
+    display: none;
+  }
+
+  .file-open {
+    grid-template-columns: 3rem minmax(0, 1fr) auto;
+  }
+
+  .file-type {
+    display: none;
+  }
+
+  .file-browser.is-list .file-card {
+    grid-template-columns: minmax(0, 1fr) minmax(7rem, auto);
+  }
+
   h1 {
     font-size: 1.65rem;
   }
@@ -213,6 +248,54 @@
     flex-wrap: wrap;
   }
 
+  .file-browser-titlebar {
+    align-items: flex-start;
+  }
+
+  .file-browser-titlebar > div:first-child {
+    flex-direction: column;
+    align-items: flex-start;
+    gap: 0.1rem;
+  }
+
+  .file-browser {
+    padding: 0.25rem;
+  }
+
+  .file-open {
+    grid-template-columns: 2.65rem minmax(0, 1fr);
+    gap: 0.55rem;
+    padding: 0.5rem;
+  }
+
+  .file-media {
+    width: 2.65rem;
+    height: 2.65rem;
+  }
+
+  .file-size {
+    display: none;
+  }
+
+  .file-browser.is-list .file-card {
+    grid-template-columns: 1fr;
+    gap: 0.25rem;
+  }
+
+  .file-browser.is-list .file-reaction-dock {
+    justify-content: flex-end;
+    padding: 0 0.5rem 0.5rem;
+  }
+
+  .file-browser.is-thumbs {
+    grid-template-columns: repeat(2, minmax(0, 1fr));
+    padding: 0.5rem;
+  }
+
+  .file-browser.is-thumbs .file-open {
+    height: 100%;
+  }
+
   .file-actions,
   .file-browser.is-thumbs .file-actions {
     width: 100%;