Frontend redesign chunk 1: design tokens and type

Replace hardcoded dark-palette Tailwind classes with semantic CSS custom
properties (page/surface/raised/line/ink/accent/sev-moderate/sev-major/
map-*) defined on :root (light) and .dark (dark), wired through
tailwind.config.ts theme.extend.colors. Add IBM Plex Sans/Mono via
next/font/google: sans for everything a person reads, mono reserved for
machine identifiers only. Drop font-mono from body and Button's base
classes. Existing !important light-mode overrides kept temporarily so
nothing goes unreadable mid-migration (removed in chunk 4).

Per UI_REDESIGN.md chunk 1.

drb-frontend/components/ui/Button.tsx

@@ -22,7 +22,7 @@ const SIZE_CLASSES: Record<Size, string> = {
 };
 
 const BASE =
-  "inline-flex items-center justify-center font-semibold font-mono transition-colors " +
+  "inline-flex items-center justify-center font-semibold transition-colors " +
   "disabled:opacity-50 disabled:cursor-not-allowed " +
   "focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-indigo-500 focus-visible:ring-offset-2 focus-visible:ring-offset-gray-950";