refactor: replace hardcoded color hex with named brand tokens (Closes #872)

[waterWang]

Summary

Replaces hardcoded hex color values with named brand tokens and CSS variables, making the app more theme-aware and reducing hardcoded colors.

Changes

tailwind.config.ts

• Added brand.blue token (rgb(var(--blue) / <alpha-value>)) for the chart blue color used in admin dashboards

app/globals.css

• Added --blue: 59 130 246 CSS variable to :root (dark theme default)

components/PlayerCard.tsx

• Replaced inline style={{ backgroundColor: '#22c55e', borderColor: '#4b5563' }} with conditional Tailwind classes (bg-green-500, border-gray-600, border-transparent)

components/admin/PlatformAnalyticsCharts.tsx

• BRAND_GREEN: #00C853 → rgb(var(--green)) (ties to theme)
• BRAND_BLUE: #3B82F6 → rgb(var(--blue)) (ties to theme)
• TOOLTIP_STYLE.backgroundColor: #111827 → rgb(var(--card)) (ties to brand-card)
• TOOLTIP_STYLE.border: #374151 → rgb(55 65 81) (documented gray-700 RGB)

components/admin/FeeRevenueChart.tsx

• Same color constant replacements as PlatformAnalyticsCharts

Snapshot updates

• Updated ScoutProfileCard snapshot to reflect the className change in PlayerCard

Verification

• npm run build — ✅ passes
• npm run test — ✅ all snapshots pass (7/7). Pre-existing failures (SQLite store tests) unchanged.

Closes #872