fix(zoom): keep the window filled when zoomed out

CSS `zoom` doesn't rescale viewport units, so the 100vh app root left an empty
strip below the UI when zoomed under 100%. Counter-size the app root to
(100/z)vw × (100/z)vh so that, after the zoom scales it back down, it exactly
fills the window again. Zoom still applies to the whole document so portalled
menus/modals scale with it.

frontend/src/App.tsx

@@ -845,13 +845,29 @@ export default function App() {
   useEffect(() => { wkEsmRef.current = wkEsm; }, [wkEsm]);
 
   // Persistent Ctrl+wheel zoom. The native WebView2 Ctrl+wheel zoom isn't saved,
-  // so we run our own: CSS `zoom` on the root element, factor stored in
+  // so we run our own: CSS `zoom` on the app root, factor stored in localStorage
-  // localStorage and restored at startup. Ctrl+0 resets to 100%.
+  // and restored at startup. Ctrl+0 resets to 100%.
+  //
+  // `zoom` scales the element uniformly (grid included) but does NOT rescale
+  // viewport units, so a 100vh/100vw root would leave an empty strip when zoomed
+  // out. We counter that by sizing the root to (100/z)vw × (100/z)vh so that,
+  // after the zoom multiplies it back down, it exactly fills the window.
+  const appRootRef = useRef<HTMLDivElement>(null);
   useEffect(() => {
     const KEY = 'opslog.uiZoom';
     let z = parseFloat(localStorage.getItem(KEY) || '1');
     if (!Number.isFinite(z) || z <= 0) z = 1;
-    const apply = () => { (document.documentElement.style as any).zoom = String(z); };
+    const apply = () => {
+      // Zoom the whole document (so portalled menus/modals/tooltips scale too)…
+      (document.documentElement.style as any).zoom = String(z);
+      // …and counter-size the app root: `zoom` doesn't rescale vh/vw, so a 100vh
+      // root leaves an empty strip when zoomed out. (100/z)vh zoomed by z = 100vh.
+      const el = appRootRef.current;
+      if (el) {
+        el.style.width = `${100 / z}vw`;
+        el.style.height = `${100 / z}vh`;
+      }
+    };
     apply();
     const onWheel = (e: WheelEvent) => {
       if (!e.ctrlKey && !e.metaKey) return; // plain wheel is left alone (scroll / freq nudge)
@@ -4034,7 +4050,7 @@ export default function App() {
   };
 
   return (
-    <div className="flex flex-col h-screen overflow-hidden bg-background">
+    <div ref={appRootRef} className="flex flex-col h-screen overflow-hidden bg-background">
       <ShutdownProgress />
       {/* ===== TOPBAR ===== */}
       {compact ? (