fix: use ResizeObserver for terminal refit after sidebar toggle

The previous transitionend approach (v0.4.5) had a { once: true } bug:
the sidebar transitions two CSS properties (width and min-width), and if
min-width fired first, the listener self-destructed before the width event
arrived — so _refitTerminal() was never called.

Replace the fragile event-based approach with a ResizeObserver on
#terminal-container. This automatically calls _fitAddon.fit() (debounced
50ms) whenever the container dimensions change, handling sidebar toggle,
browser resize, and any future layout change. Remove the now-unnecessary
window._refitTerminal global and the broken transitionend handler.

Bumps version to 0.4.6.

muxplex/frontend/app.js

@@ -804,13 +804,6 @@ function toggleSidebar() {
 
   var collapseBtn = $('sidebar-collapse-btn');
   if (collapseBtn) collapseBtn.textContent = isOpen ? '\u2039' : '\u203a';
-
-  // Refit xterm.js after the sidebar CSS transition completes (0.25s ease).
-  // 'width' fires on desktop (flex layout); 'transform' fires on mobile (overlay).
-  sidebar.addEventListener('transitionend', function(e) {
-    if (e.propertyName !== 'width' && e.propertyName !== 'transform') return;
-    if (window._refitTerminal) window._refitTerminal();
-  }, { once: true });
 }
 
 /**

muxplex/frontend/terminal.js

@@ -10,6 +10,7 @@ let _currentSession = null;
 let _vpHandler = null;
 let _reconnectAttempts = 0; // tracks consecutive failed reconnect attempts for backoff + ttyd respawn
 let _searchAddon = null;
+let _resizeObserver = null;
 
 // ─── Module-level encoding helpers ──────────────────────────────────────────
 // Hoisted here so the clipboard key handler (in openTerminal) can also use them.
@@ -357,6 +358,23 @@ function openTerminal(sessionName, remoteId, fontSize) {
 
   _term.open(container);
 
+  // --- Auto-refit on container resize (sidebar toggle, etc.) ---
+  // xterm.js FitAddon only resizes on explicit fit() calls. A ResizeObserver
+  // on the container handles ALL layout changes: sidebar toggle, window resize,
+  // and any future CSS geometry change. Debounced to coalesce rapid events
+  // (e.g. during CSS transition animation frames).
+  if (_resizeObserver) { _resizeObserver.disconnect(); _resizeObserver = null; }
+  if (typeof ResizeObserver !== 'undefined') {
+    var _roTimer = null;
+    _resizeObserver = new ResizeObserver(function() {
+      clearTimeout(_roTimer);
+      _roTimer = setTimeout(function() {
+        if (_fitAddon) try { _fitAddon.fit(); } catch (_) {}
+      }, 50);
+    });
+    _resizeObserver.observe(container);
+  }
+
   // --- Clipboard integration ---
   // Copy: Ctrl+Shift+C intercepts and copies selection to system clipboard
   // Paste: handled natively by xterm.js (browser paste event → hidden textarea → onData → WebSocket)
@@ -504,6 +522,8 @@ function closeTerminal() {
     _ws = null;
   }
 
+  if (_resizeObserver) { _resizeObserver.disconnect(); _resizeObserver = null; }
+
   if (_term) {
     _term.dispose();
     _term = null;
@@ -542,13 +562,6 @@ function setTerminalFontSize(size) {
 
 window._setTerminalFontSize = setTerminalFontSize;
 
-// Refit the terminal to its container dimensions.
-// Call after layout changes (e.g. sidebar toggle) that alter available space.
-window._refitTerminal = function() {
-  if (!_fitAddon) return;
-  try { _fitAddon.fit(); } catch (_) {}
-};
-
 // ---------------------------------------------------------------------------
 // Mobile touch scroll — rAF-batched WheelEvent dispatch
 // Mobile devices batch touchmove events irregularly; dispatching one WheelEvent