fix: fit view — reapply layout on session close, show more lines, bottom-anchor content

Bug 1: closeSession() didn't call applyFitLayout when returning to
dashboard — tiles kept old heights from before the overview was hidden.
Fix: add rAF call after overview.style.display = '' so the layout is
remeasured once the overview becomes visible again.

Bug 2: Only 20 lines shown in tiles — not enough for tall fit-mode
tiles, and content sat at the top. Fix:
  - buildTileHTML shows 80 lines in fit mode (vs 20 in auto)
  - renderGrid rAF sets pre.scrollTop = pre.scrollHeight after
    applyFitLayout so content anchors at the bottom (like a real
    terminal). Hidden scrollbar via scrollbar-width:none + ::-webkit.

Tests: 3 new source-scan tests added and verified RED before GREEN.

muxplex/frontend/style.css

@@ -1380,10 +1380,16 @@ body {
   overflow: hidden;
 }
 
-/* In fit mode, stretch pre to fill the full tile body height (not just bottom-anchor) */
+/* In fit mode, stretch pre to fill the full tile body height and anchor content to bottom */
 .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 */
+  top: 0;                   /* stretch to fill full tile body height in fit mode */
+  overflow-y: scroll;       /* enable scrollTop positioning (content anchored via JS scrollTop=scrollHeight) */
+  scrollbar-width: none;    /* Firefox: hide scrollbar */
+  -ms-overflow-style: none; /* IE/Edge: hide scrollbar */
+}
+
+.session-grid--fit .tile-body pre::-webkit-scrollbar {
+  display: none;            /* Chrome/Safari: hide scrollbar */
 }
 
 /* ============================================================