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/PageHeader.tsx

@@ -13,10 +13,10 @@ export function PageHeader({ title, description, badge, action }: PageHeaderProp
     <div className="flex flex-col sm:flex-row sm:items-start sm:justify-between gap-3">
       <div className="min-w-0">
         <div className="flex items-center gap-3 flex-wrap">
-          <h1 className="text-xl font-bold text-white font-mono">{title}</h1>
+          <h1 className="text-xl font-semibold text-ink">{title}</h1>
           {badge}
         </div>
-        {description && <p className="text-gray-500 text-sm mt-1 max-w-2xl">{description}</p>}
+        {description && <p className="text-ink-muted text-sm mt-1 max-w-2xl">{description}</p>}
       </div>
       {action && <div className="shrink-0">{action}</div>}
     </div>