feat: touch scroll for terminal and sidebar on mobile

Terminal: add touchstart/touchmove/touchend IIFE on #terminal-container.
Vertical swipe delta translated to _term.scrollLines(n). touchmove is
non-passive (required for e.preventDefault to block page scroll while
swiping inside terminal). mouse wheel scroll unaffected (separate wheel
event path in xterm.js).

Sidebar: add touch-action:pan-y + overscroll-behavior:contain to
.sidebar-list. Tells browser explicitly that vertical panning is allowed
and prevents scroll chaining when reaching list ends.

muxplex/frontend/style.css

@@ -427,6 +427,8 @@ body {
   display: flex;
   flex-direction: column;
   gap: 6px;
+  touch-action: pan-y;          /* tell browser vertical panning is allowed on mobile */
+  overscroll-behavior: contain; /* prevent scroll chaining to non-scrollable parent */
 }
 
 .sidebar-collapse-btn {