Add hillshade and traffic overlay layers with layer control UI

- New LayerControl component with popover toggles for hillshade/traffic
- MapView: add/remove hillshade raster-dem and traffic raster layers
- Overlay layers persist in localStorage, survive theme swaps
- Hillshade defaults ON, traffic defaults OFF when available

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/index.css

@@ -282,3 +282,103 @@ body {
   transform: translateX(0);
   opacity: 1;
 }
+
+/* ═══ LAYER CONTROL ═══ */
+.layer-control {
+  position: absolute;
+  bottom: 32px;
+  right: 10px;
+  z-index: 10;
+}
+
+.layer-control-btn {
+  width: 36px;
+  height: 36px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  background: var(--bg-raised);
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  color: var(--text-secondary);
+  cursor: pointer;
+  box-shadow: var(--shadow);
+  transition: color 0.1s, border-color 0.1s;
+}
+
+.layer-control-btn:hover {
+  color: var(--text-primary);
+  border-color: var(--accent);
+}
+
+.layer-control-popover {
+  position: absolute;
+  bottom: 44px;
+  right: 0;
+  min-width: 160px;
+  background: var(--bg-raised);
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  padding: 8px 0;
+  box-shadow: var(--shadow-lg);
+}
+
+.layer-control-header {
+  padding: 4px 12px 6px;
+  font-size: var(--text-xs);
+  font-weight: 600;
+  color: var(--text-tertiary);
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+}
+
+.layer-control-item {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: 6px 12px;
+  cursor: pointer;
+  transition: background 0.1s;
+}
+
+.layer-control-item:hover {
+  background: var(--bg-overlay);
+}
+
+.layer-control-label {
+  font-size: var(--text-sm);
+  color: var(--text-primary);
+}
+
+.layer-control-toggle {
+  appearance: none;
+  width: 32px;
+  height: 18px;
+  background: var(--border);
+  border-radius: 9px;
+  position: relative;
+  cursor: pointer;
+  transition: background 0.15s;
+  flex-shrink: 0;
+  margin-left: 12px;
+}
+
+.layer-control-toggle::after {
+  content: '';
+  position: absolute;
+  top: 2px;
+  left: 2px;
+  width: 14px;
+  height: 14px;
+  background: var(--text-primary);
+  border-radius: 50%;
+  transition: transform 0.15s;
+}
+
+.layer-control-toggle:checked {
+  background: var(--accent);
+}
+
+.layer-control-toggle:checked::after {
+  transform: translateX(14px);
+}