fix: fit view — defer layout measurement to rAF, stretch pre to fill tile

Bug 1: applyFitLayout measured parentElement.clientHeight before the
browser reflowed the DOM — returned 0 on first render. Fix: wrap in
requestAnimationFrame so measurement happens after layout pass.
Applies to all three call sites: renderGrid(), applyDisplaySettings(),
and the window resize handler.

Bug 2: .tile-body pre had position:absolute bottom:0 but no top:0.
In fit mode (taller tiles), content sat at the bottom with a gap above.
Fix: .session-grid--fit .tile-body pre { top: 0; overflow-y: auto }

muxplex/frontend/style.css

@@ -1380,6 +1380,12 @@ body {
   overflow: hidden;
 }
 
+/* In fit mode, stretch pre to fill the full tile body height (not just bottom-anchor) */
+.session-grid--fit .tile-body pre {
+  top: 0;           /* stretch to fill full tile body height in fit mode */
+  overflow-y: auto; /* scroll if content exceeds the tile */
+}
+
 /* ============================================================
    Responsive overlay sidebar at <960px
    ============================================================ */