fix(zoom): use transform:scale instead of CSS zoom to kill map tile seams

CSS `zoom` re-lays-out and pixel-rounds each element, which opened ~1px seams
between map tiles (a faint grid over the map when zoomed). Switch the persistent
UI zoom to `transform: scale(z)` from the app root's top-left, with the root
counter-sized to (100/z)vw × (100/z)vh so it reflows to fill and scales back to
the window. transform scales the subtree as one composited layer, so tiles stay
seamless. Removed the now-unneeded tile-overlap CSS hack.

frontend/src/style.css

@@ -652,13 +652,3 @@
   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;
-}