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

templates/league_session_view.html

@@ -1,7 +1,7 @@
 {% extends "base.html" %}
 {% block title %}{{ session_model.display_label }} · {{ league.name }} · myboker.org{% endblock %}
-{% block subnav %}{% include "_league_subnav.html" %}{% endblock %}
-{% block page_class %}page--session{% endblock %}
+{% block sidebar %}{% include "_league_sidebar.html" %}{% endblock %}
+{% block page_class %}page--app{% endblock %}
 {% block content %}
 
 <div class="breadcrumb">