feat: apply settings effects — font size, grid columns, hidden sessions, sort order

- terminal.js createTerminal(): read fontSize from localStorage 'muxplex.display',
  default 14, apply Math.min(storedFontSize, 12) mobile cap
- app.js renderGrid(): filter hidden sessions via _serverSettings.hidden_sessions,
  create visible array, apply alphabetical sort via localeCompare when sort_order is
  'alphabetical'; 'recent'/'manual' use server-provided order
- app.js renderSidebar(): apply same hidden sessions filter, use visible array
- app.js DOMContentLoaded: call loadServerSettings() after startPolling() in restoreState().then()
- tests: 13 new tests in test_frontend_js.py covering all spec requirements

Co-authored-by: Amplifier <amplifier@example.com>

muxplex/frontend/terminal.js

@@ -140,11 +140,22 @@ function createTerminal() {
     _fitAddon = null;
   }
 
+  // Read font size from display settings (localStorage key 'muxplex.display')
+  var storedFontSize = 14;
+  try {
+    var raw = localStorage.getItem('muxplex.display');
+    if (raw) {
+      var parsed = JSON.parse(raw);
+      if (parsed && parsed.fontSize) storedFontSize = parsed.fontSize;
+    }
+  } catch (_) { /* use default 14 */ }
+
   const mobile = window.innerWidth < 600;
+  const fontSize = mobile ? Math.min(storedFontSize, 12) : storedFontSize;
 
   _term = new window.Terminal({
     cursorBlink: true,
-    fontSize: mobile ? 12 : 14,
+    fontSize: fontSize,
     fontFamily: "'SF Mono', 'Fira Code', Consolas, monospace",
     theme: {
       background: '#000000',