feat: three dashboard view modes — Auto, Fit, Compact

Auto: current behavior (auto-fill grid, fixed tile height, scrollable).
Fit: calculates cols × rows to fill viewport exactly, zero scroll.
Compact: 80px tiles, high density, auto-fill with 200px min-width.

Toggle button (▦) in overview header cycles modes. Fit recalculates
on window resize. View mode persisted in localStorage alongside other
display settings (viewMode: 'auto' default in DISPLAY_DEFAULTS).

- CSS: .session-grid--fit and .session-grid--compact modifiers
- JS: VIEW_MODES, cycleViewMode(), applyFitLayout() functions
- HTML: #view-mode-btn between + and ⚙ in overview header
- Tests: 8 new JS tests, 3 CSS tests, 1 HTML test all passing

muxplex/frontend/style.css

@@ -1371,6 +1371,24 @@ body {
   z-index: 200;
 }
 
+/* ============================================================
+   Dashboard view modes — Compact and Fit
+   ============================================================ */
+
+/* Compact view — small tiles, high density */
+.session-grid--compact .session-tile {
+  height: 80px;
+}
+
+.session-grid--compact .tile-body {
+  height: 40px;
+}
+
+/* Fit view — tiles fill viewport, no scroll */
+.session-grid--fit {
+  overflow: hidden;
+}
+
 /* ============================================================
    Responsive overlay sidebar at <960px
    ============================================================ */