feat: loading placeholder tile while new session is being created

- Inject .tile--loading skeleton tile into #session-grid immediately after
  POST /api/sessions succeeds, giving instant visual feedback while the
  backend creates the tmux session (which may take several seconds)
- Remove the placeholder when the poll finds the session or on timeout
- Add shimmer animation via @keyframes shimmer to style.css
- Tile placed after original .tile-body pre rule to avoid selector-substring
  match in _extract_rule_block CSS tests

muxplex/frontend/terminal.js

@@ -258,6 +258,26 @@ function closeTerminal() {
 window._openTerminal = openTerminal;
 window._closeTerminal = closeTerminal;
 
+// ---------------------------------------------------------------------------
+// setTerminalFontSize — live font-size update without reconnecting
+// ---------------------------------------------------------------------------
+
+/**
+ * Update the terminal font size at runtime without reconnecting.
+ * Modifies _term.options.fontSize and refits the terminal to recalculate dimensions.
+ * No-op when no terminal is open.
+ * @param {number} size - font size in pixels
+ */
+function setTerminalFontSize(size) {
+  if (!_term) return;
+  _term.options.fontSize = size;
+  if (_fitAddon) {
+    try { _fitAddon.fit(); } catch (_) {}
+  }
+}
+
+window._setTerminalFontSize = setTerminalFontSize;
+
 // ---------------------------------------------------------------------------
 // Android touch scroll — rAF-batched WheelEvent dispatch
 // Android batches touchmove events irregularly; dispatching one WheelEvent