Frontend redesign chunk 2: primitives and marks

Rewrite components/ui/* (Button, Card, Badge, PageHeader, EmptyState,
Skeleton) against the chunk-1 tokens instead of hardcoded gray-9xx classes,
and drop the remaining font-mono from label/heading text.

Add the three colour-blindness-validated encoding components from
UI_REDESIGN.md §2.3:
- components/marks/SeverityMark.tsx — glyph (filled triangle / outline
  triangle / outline circle) + optional spine + optional label, from the
  four-level severity ladder. Colour is never the only channel.
- components/marks/TypeGlyph.tsx — five stroked SVG glyphs (fire, police,
  ems, collision, other) in currentColor. Incident type is now shape, not
  hue, since five hues can't clear an all-pairs CVD gate.
- components/marks/NodeMark.tsx — diamond at four weights (filled+ring /
  filled / hollow / hollow-dashed). Green is gone from node state entirely.

lib/severity.tsx now renders through SeverityMark; SEVERITY_COLORS reads
the validated sev-moderate/sev-major tokens with routine/minor neutral.
IncidentBadges.tsx's TypeBadge is reimplemented on TypeGlyph instead of a
coloured pill.

Per UI_REDESIGN.md chunk 2.

drb-frontend/components/ui/Skeleton.tsx

@@ -1,12 +1,12 @@
 /** Loading placeholder block. Use instead of a bare "Loading…" string wherever the eventual
  *  content has a predictable shape (cards, table rows, stat tiles). */
 export function Skeleton({ className }: { className?: string }) {
-  return <div className={`skeleton bg-gray-800 rounded-md ${className ?? "h-4 w-full"}`} />;
+  return <div className={`skeleton bg-raised rounded-md ${className ?? "h-4 w-full"}`} />;
 }
 
 export function SkeletonCard() {
   return (
-    <div className="bg-gray-900 border border-gray-800 rounded-xl p-4 space-y-3">
+    <div className="bg-surface border border-line rounded-xl p-4 space-y-3">
       <Skeleton className="h-4 w-1/3" />
       <Skeleton className="h-3 w-2/3" />
       <Skeleton className="h-3 w-1/2" />
@@ -16,7 +16,7 @@ export function SkeletonCard() {
 
 export function SkeletonRow({ cols = 5 }: { cols?: number }) {
   return (
-    <tr className="border-b border-gray-800">
+    <tr className="border-b border-line">
       {Array.from({ length: cols }).map((_, i) => (
         <td key={i} className="px-4 py-3">
           <Skeleton className="h-3 w-full max-w-[8rem]" />