fix(zoom): hide Leaflet tile seams that appear when the map is CSS-zoomed

Under our CSS zoom the map tiles scale by a fractional factor, opening ~1px gaps
between adjacent tiles that read as a faint grid over the map. Tag the document
with data-uizoom and, only while zoomed, enlarge each 256px tile by 1px so
neighbours overlap and cover the seams. The 100% view is untouched.

frontend/src/App.tsx

@@ -860,6 +860,9 @@ export default function App() {
     const apply = () => {
       // Zoom the whole document (so portalled menus/modals/tooltips scale too)…
       (document.documentElement.style as any).zoom = String(z);
+      // Flag the zoom state so the Leaflet tile-seam fix (style.css) only kicks in
+      // when actually zoomed (fractional scaling opens 1px gaps between tiles).
+      document.documentElement.setAttribute('data-uizoom', 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;

frontend/src/style.css

@@ -652,3 +652,13 @@
   border: 2px solid var(--background);
 }
 ::-webkit-scrollbar-thumb:hover { background: var(--scrollbar-thumb-hover); }
+
+/* When the UI is CSS-zoomed (our persistent Ctrl+wheel zoom), map tiles are
+   scaled by a fractional factor, which opens ~1px seams between adjacent tiles
+   (visible as a faint grid over the map). Enlarging each 256px tile by 1px makes
+   neighbours overlap just enough to hide the seams. Scoped to the zoomed state so
+   it never touches the crisp 100% view. */
+html[data-uizoom]:not([data-uizoom="1"]) .leaflet-tile {
+  width: 257px !important;
+  height: 257px !important;
+}