Stream A: DW brand theme — Instrument Sans + IBM Plex Mono, light/dark mode, all hardcoded colors replaced

- Replace Solar Momentum palette with DiligenceWorks brand identity
- Typography: Outfit → Instrument Sans, Plus Jakarta Sans → IBM Plex Mono (data/stats)
- CSS variables: --orange* → --accent*, new DW blue #2952CC / #6B9BFF
- Dark mode: prefers-color-scheme media query + manual data-theme override
- Border radii: 14/10/20px → 8/6/12px (professional, less playful)
- Font weights: 800/900 → 600/700 (DW uses lighter weights)
- Fixed 30+ hardcoded hex values across 16 files (UI-06)
- Gate banners, progress bars, category chips all use CSS variables

frontend/src/pages/Onboarding.jsx

@@ -198,7 +198,7 @@ export default function Onboarding() {
   return (
     <div className="page" style={{ paddingTop: '40px' }}>
       <div className="progress-bar" style={{ marginBottom: '24px' }}>
-        <div className="progress-bar-fill" style={{ width: `${progress}%`, background: 'var(--orange)' }} />
+        <div className="progress-bar-fill" style={{ width: `${progress}%`, background: 'var(--accent)' }} />
       </div>
 
       {/* Step 0: Goal */}