fix(feed): drop the compact row's columns by the ROW's width, not the viewport

The viewport was the wrong signal: the nav rail and filter panel move the
row's width by ~430px between them, so at a 1280 window with both pinned
every column still qualified while the row had 762px for 766px of them —
the title squeezed back to nothing, the exact bug the breakpoints were
meant to fix. My own comment predicted it and I didn't follow it through.

VirtualFeed already measures the container for colCount, so it now passes
the item's real width down. Measured in the failing config: title -4 ->
359 with the date column standing down.

frontend/src/components/VirtualFeed.tsx

@@ -86,6 +86,12 @@ export default function VirtualFeed({
   // Distance from the scroll element's content top to where this list starts (the
   // feed toolbar sits above it inside the same scroll area).
   const [scrollMargin, setScrollMargin] = useState(0);
+  // How wide ONE row/card actually renders. The one-line row drops columns as this shrinks, and it
+  // has to be the real width, not the viewport: the nav rail (56 slim / ~230 pinned) and the filter
+  // panel (~80 tab / ~256 pinned) move it by ~430px between them. A viewport breakpoint got this
+  // wrong — at 1280 with both pinned every column still showed while the row had 762px for 766px of
+  // them, which squeezed the title to nothing. This is measured here anyway, for colCount.
+  const [itemWidth, setItemWidth] = useState(0);
 
   useLayoutEffect(() => {
     if (listRef.current) setScrollEl(getScrollParent(listRef.current));
@@ -103,7 +109,13 @@ export default function VirtualFeed({
           scrollEl.scrollTop;
         setScrollMargin(m);
       }
-      setColCount(columnsFor(view, node.clientWidth));
+      const w = node.clientWidth;
+      const cols = columnsFor(view, w);
+      setColCount(cols);
+      const { minCol: min, maxCol: max } = FEED_VIEW_SPEC[view];
+      setItemWidth(
+        min == null ? Math.min(w, max ?? w) : Math.floor((w - GRID_GAP * (cols - 1)) / cols)
+      );
     };
     measure();
     const ro = new ResizeObserver(measure);
@@ -168,6 +180,7 @@ export default function VirtualFeed({
                   key={v.id}
                   video={v}
                   view={view}
+                  width={itemWidth}
                   onState={onState}
                   onToggleSave={onToggleSave}
                   onOpenChannel={onOpenChannel}