visual redesign: landing, dashboard overview, session calendar (2.3.1)

- Redesign landing page hero with split layout, features section, and in-app session/result demos
- Add fixed sidebar scroll isolation (app-shell overflow lock pattern)
- Redesign leagues index cards with initial badge, podium leader chips, and action links
- Redesign league dashboard: color-coded nav tiles, cash/stats overview strip, session history heatmap calendar
- Dashboard cash stats use real cash only (excludes rollovers and carry-ins)
- Session history calendar: JS-driven full-year grid, year navigation, hover tooltips, full-width responsive layout
- Bump leaderboard tile color from gold to blue (distinct from ledger orange)
- Bump version to 2.3.1

static/favicon.svg

@@ -1,4 +1,4 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 26 26">
   <rect width="26" height="26" rx="5" fill="#9b8cf0"/>
-  <path d="M13 5.22 20.78 13 13 20.78 5.22 13Z" fill="#0f0f12"/>
+  <path d="M13 5.22 20.78 13 13 20.78 5.22 13Z" fill="#12101e"/>
 </svg>