fix(explore): bound the 3-pane grid row so a tall rail can't scroll the page

The panes grid had no explicit row, so the implicit `auto` row sized to its
tallest pane's content. With Provenance + Tags + a long Suggestions list, the
rail outgrew the fixed-height workspace, spilled over and made the WHOLE page
scrollable — showing as a weird empty gap at the top (operator-flagged
2026-06-26). grid-template-rows: minmax(0, 1fr) bounds the row to the container
so each pane's own overflow-y:auto scrolls internally instead. Reset to `none`
in the stacked (<=1100px) layout where the page is meant to scroll.

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

frontend/src/views/ExploreView.vue

@@ -235,11 +235,17 @@ onUnmounted(() => document.removeEventListener('keydown', onKeyDown))
 .fc-ex__crumb:focus-visible { outline: 2px solid rgb(var(--v-theme-accent)); outline-offset: 1px; }
 .fc-ex__reseed { margin-left: auto; }
 
-/* The three panes fill the remaining height; each scrolls on its own. */
+/* The three panes fill the remaining height; each scrolls on its own.
+   grid-template-rows: minmax(0, 1fr) BOUNDS the single row to the container
+   height — without it the row is `auto` and a tall pane (Provenance + Tags +
+   a long Suggestions list) grows the row past the fixed-height view, spilling
+   over and scrolling the whole page (the weird top gap). Bounded, the panes'
+   own overflow-y:auto kicks in and they scroll internally instead. */
 .fc-ex__panes {
   flex: 1 1 auto; min-height: 0;
   display: grid;
   grid-template-columns: 340px minmax(0, 1fr) var(--fc-side-w, 320px);
+  grid-template-rows: minmax(0, 1fr);
   gap: 1px;
   background: rgb(var(--v-theme-surface-light));
 }
@@ -303,6 +309,8 @@ onUnmounted(() => document.removeEventListener('keydown', onKeyDown))
   .fc-ex { height: auto; }
   .fc-ex__panes {
     grid-template-columns: 1fr;
+    /* Stacked: let each pane take its natural height and the page scroll. */
+    grid-template-rows: none;
     background: transparent;
   }
   .fc-ex__viewer { order: -1; min-height: 60vh; }