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/index.html

@@ -22,6 +22,7 @@
       <h1 class="app-wordmark"><img src="/wordmark-on-dark.svg" alt="muxplex" height="24" /></h1>
       <div class="header-actions">
         <button id="new-session-btn" class="header-btn" aria-label="New session">+</button>
+        <button id="view-mode-btn" class="header-btn" aria-label="Toggle view mode" title="View: auto">&#9638;</button>
         <button id="settings-btn" class="header-btn" aria-label="Settings">&#9881;</button>
         <span id="connection-status"></span>
       </div>