Make admin panel usable on mobile

The sidebar was a fixed 220px flex column that left barely any room
for content on a phone, and tables had no min-width so columns just
squeezed into unreadable slivers instead of scrolling.

Moves the sidebar's layout rules out of inline styles (which media
queries can't override) into real CSS classes, and collapses it into
a horizontally-scrollable pill bar below 768px so the content area
gets the full screen width. Tables now get a sensible min-width so
they scroll horizontally on narrow screens instead of squishing, and
the per-player tab bar on the session log view scrolls too.

templates/game/admin/sessions/index.html.twig

@@ -9,7 +9,7 @@
     </div>
 
     <div style="background: #fff; border-radius: 8px; box-shadow: 0 1px 3px rgba(0,0,0,.07); overflow: hidden;">
-        <table style="width: 100%; border-collapse: collapse; font-size: 0.9rem;">
+        <table style="width: 100%; min-width: 700px; border-collapse: collapse; font-size: 0.9rem;">
             <thead>
                 <tr style="background: #f1f5f9; border-bottom: 1px solid #e2e8f0;">
                     <th style="padding: 0.75rem 1rem; text-align: left; color: #475569; font-weight: 600;">ID</th>