web: undo/redo, a clean preview, and a histogram that stays put

Four things the studio owed the visitor:
- UNDO/REDO in the header, so a look can be taken back and put back without
  reloading the photo; a fresh edit clears the redo trail.
- Opening a saved frame, or picking a look out of its history, now drops the
  stale preview buffer instead of leaving the previous render on the stage.
- The picked history look is marked in the accent, so it is plain which look
  the photo is wearing.
- The histogram is re-clamped against the photo box on resize, so opening a
  chip column no longer pushes the overlay past the edge of the canvas.

The 'NEW SAVES: FILM STRIP' chip goes: a save already lands in the strip.

docker/frontend/src/ui/Histogram.tsx

@@ -108,10 +108,19 @@ export function Histogram({ url, onClose }: { url: string; onClose: () => void }
     setPos({ x: Math.max(8, b.width - box.getBoundingClientRect().width - 12), y: 12 });
   }, [pos]);
 
+  // The photo's own box is what the frame is parked against, and it shrinks
+  // when a column opens beside the stage — not only when the window does. So
+  // the frame watches that box, not the window, and is pulled back inside it.
   useEffect(() => {
     const onResize = () => setPos((p) => (p ? clamp(p.x, p.y) : p));
     window.addEventListener('resize', onResize);
-    return () => window.removeEventListener('resize', onResize);
+    const wrap = boxRef.current?.parentElement;
+    const ro = wrap ? new ResizeObserver(onResize) : null;
+    ro?.observe(wrap as Element);
+    return () => {
+      window.removeEventListener('resize', onResize);
+      ro?.disconnect();
+    };
   }, [clamp]);
 
   const start = (e: React.PointerEvent<HTMLDivElement>) => {