fix(map): readable incident rail — dark basemap, opaque cards, AA colours (#136)

- "Dark" base layer was bright OSM in prod; invert the raster via
  .drb-tiles-dark so dark-theme overlays sit on a dark ground.
- Rail and mobile incident cards are opaque, not glass over the map.
- Dark --ink-muted #77839A -> #8C98AE, --sev-major #D03B3B -> #F05252
  (both failed AA as small text on surface/raised).
- Card borders used `color + "55"` on CSS var() colours, which is
  invalid CSS — no border rendered. Now color-mix.
- 10px rail labels -> 11px.

Frontend unverified: no Node on the authoring box (#207).

Closes #136

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-frontend/app/globals.css

@@ -43,11 +43,11 @@
   --line-strong: rgba(255,255,255,.17);
   --ink:         #E8EBF0;
   --ink-2:       #A5B0C2;
-  --ink-muted:   #77839A;
+  --ink-muted:   #8C98AE; /* was #77839A: 4.2:1 on raised, failed AA at the 10-12px it's used at (#136) */
 
   --accent:       #3987E5;
   --sev-moderate: #C98500;
-  --sev-major:    #D03B3B;
+  --sev-major:    #F05252; /* was #D03B3B: 3.7:1 as text on surface (#136) */
 
   --map-bg:    #0E131B;
   --map-block: #161C26;
@@ -191,6 +191,14 @@ html:not(.dark) .border-indigo-800 { border-color: #a5b4fc !important; }
   z-index: 0;
 }
 
+/* "Dark" base layer (#136): the prod tile URL is bright OSM, so dark-theme
+ * overlay cards sat on a bright, busy map. Inverting the raster keeps the
+ * keyless OSM source (CARTO's dark_all CDN proved flaky, #117) while giving
+ * the overlays the dark ground their palette was chosen for. */
+.leaflet-tile-pane .drb-tiles-dark {
+  filter: invert(1) hue-rotate(180deg) brightness(0.9) contrast(0.9) saturate(0.6);
+}
+
 /* Pinned 511 camera tiles (MapView DotCameraLayer): strip Leaflet's tooltip
  * padding and let the tile's own theme colours show, so the tile is no bigger
  * than the video it holds. */