Swap flag emoji for real (square, bordered) SVG flags

Emoji flags are font glyphs, not shapes — no way to resize, border, or
tone them down to fit this app's controlled, hand-drawn-icon look
(same reasoning the topbar readout already settled earlier this
session). Vendored flag-icons' pre-cropped square SVGs (MIT) instead,
framed in a small rounded-square badge — the same convention
icons.js's MAP_MARKER_ICON already uses for framing a glyph, not a new
visual language.

public/css/style.css

@@ -428,6 +428,18 @@ html, body {
   background: #f0f0f0;
 }
 
+/* Same rounded-square badge ratio as icons.js's own MAP_MARKER_ICON
+   (rx 5 of a 22-wide square) — a consistent frame rather than however
+   each flag's own artwork happens to fill an unbordered square. */
+.offline-data-panel__flag {
+  flex-shrink: 0;
+  width: 18px;
+  height: 18px;
+  border-radius: 4px;
+  border: 1px solid rgba(0, 0, 0, 0.15);
+  object-fit: cover;
+}
+
 .offline-data-panel__region-status {
   padding: 6px 8px;
   color: #888;