refactor: code quality improvements for task-7 settings effects

- Extract getVisibleSessions() helper to consolidate hidden-session
  filter used by renderGrid() and renderSidebar() (DRY reduction)
- Add comment to terminal.js magic literal 600 noting it matches
  MOBILE_THRESHOLD in app.js
- Move TERMINAL_JS_PATH and _TERMINAL_JS module fixtures to the top
  of the test file alongside JS_PATH and _JS for consistency
- Strengthen test_render_grid_uses_visible_for_empty_state_check to
  assert visible.length specifically, distinguishing it from
  test_render_grid_creates_visible_array
- Update renderGrid/renderSidebar filter tests to assert getVisibleSessions()
  call; add test_get_visible_sessions_filters_hidden_sessions to cover
  the extracted helper directly

muxplex/frontend/app.js

@@ -448,6 +448,17 @@ function buildSidebarHTML(session, currentSession) {
   );
 }
 
+/**
+ * Returns sessions with hidden session names removed.
+ * Consolidates the hidden-session filter used by all render paths.
+ * @param {object[]} sessions
+ * @returns {object[]}
+ */
+function getVisibleSessions(sessions) {
+  const hidden = (_serverSettings && _serverSettings.hidden_sessions) || [];
+  return (sessions || []).filter((s) => !hidden.includes(s.name));
+}
+
 /**
  * Render the session sidebar list. Only renders in fullscreen view.
  * Shows empty state when no sessions exist.
@@ -461,9 +472,7 @@ function renderSidebar(sessions, currentSession) {
   const list = $('sidebar-list');
   if (!list) return;
 
-  // Filter hidden sessions
-  const hiddenSessions = (_serverSettings && _serverSettings.hidden_sessions) || [];
-  const visible = (sessions || []).filter((s) => !hiddenSessions.includes(s.name));
+  const visible = getVisibleSessions(sessions);
 
   if (visible.length === 0) {
     list.innerHTML = '<div class="sidebar-empty">No sessions</div>';
@@ -594,9 +603,7 @@ function renderGrid(sessions) {
   const grid = $('session-grid');
   const emptyState = $('empty-state');
 
-  // Filter hidden sessions
-  const hiddenSessions = (_serverSettings && _serverSettings.hidden_sessions) || [];
-  const visible = (sessions || []).filter((s) => !hiddenSessions.includes(s.name));
+  const visible = getVisibleSessions(sessions);
 
   if (visible.length === 0) {
     if (grid) grid.innerHTML = '';

muxplex/frontend/terminal.js

@@ -150,7 +150,7 @@ function createTerminal() {
     }
   } catch (_) { /* use default 14 */ }
 
-  const mobile = window.innerWidth < 600;
+  const mobile = window.innerWidth < 600; // matches MOBILE_THRESHOLD in app.js
   const fontSize = mobile ? Math.min(storedFontSize, 12) : storedFontSize;
 
   _term = new window.Terminal({