fix(modal): mobile — no tag autofocus + sticky image over scrolling panel

Operator-flagged 2026-06-05 (mobile):
- TagAutocomplete no longer autofocuses on the ≤900px stacked layout —
  focusing popped the soft keyboard, shrinking the viewport and shoving the
  pinned image + nav/close controls out of view. matchMedia gate (safe on
  plain HTTP); desktop autofocus unchanged.
- ImageViewer stacked layout: the body now scrolls and the media pane is
  sticky (height:55vh, top:0), so the image + prev/next/close + integrity
  badge stay pinned while the metadata panel scrolls beneath. Replaces the
  old fixed image + 40vh internally-scrolled panel that could push the
  controls off. Prev/next re-centered over the image band; opaque obsidian
  bg so the scrolling panel doesn't bleed through.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/modal/ImageViewer.vue

@@ -203,14 +203,37 @@ function nextFrame() {
 }
 
 @media (max-width: 900px) {
-  .fc-viewer__body { flex-direction: column; }
-  /* Side panel drops below the image — the next arrow uses the full width. */
-  .fc-viewer__nav--next { right: 16px; }
+  /* Stacked layout (operator-flagged 2026-06-05): pin the image pane and
+     its controls at the top while the metadata panel scrolls beneath it.
+     Previously the image + a 40vh panel shared one fixed viewport and the
+     controls could be pushed out of view; now the body scrolls and the
+     media is sticky, so the image + prev/next/close (and the integrity
+     badge) stay visible no matter how far the panel is scrolled. */
+  .fc-viewer__body {
+    flex-direction: column;
+    overflow-y: auto;
+  }
+  .fc-viewer__media {
+    flex: none;
+    height: 55vh;
+    position: sticky;
+    top: 0;
+    z-index: 1;
+    /* Opaque obsidian so the scrolling panel never bleeds through the
+       haze behind the pinned image. */
+    background: rgb(20, 23, 26);
+  }
   .fc-viewer__side {
     width: 100%;
-    max-height: 40vh;
+    max-height: none;
     border-left: none;
     border-top: 1px solid rgb(var(--v-theme-surface-light));
   }
+  /* Re-center the prev/next arrows over the 55vh image band (their base
+     top:50% would land on the scrolling panel); next uses the full width
+     now that the panel is below, not beside. Close + integrity badge keep
+     their top:16px/72px — already within the pinned band. */
+  .fc-viewer__nav { top: 27.5vh; }
+  .fc-viewer__nav--next { right: 16px; }
 }
 </style>