feat: add search (Ctrl+F) and image rendering (Sixel/Kitty) addons

Search: xterm-addon-search (0.13.0) with slide-down search bar. Ctrl+F
opens bar; type to search (live highlight); Enter/Shift+Enter for
next/prev; Escape to close. Bar sits above terminal-container inside
a new .terminal-wrapper flex column.

Image: xterm-addon-image (0.5.0) for inline graphics. Enables Sixel,
iTerm2 IIP, and Kitty graphic protocols — needed for tools like yazi
file manager. Auto-detects escape sequences, no configuration needed.

muxplex/frontend/style.css

@@ -631,6 +631,14 @@ body {
   text-align: center;
 }
 
+.terminal-wrapper {
+  flex: 1;
+  min-width: 0;
+  overflow: hidden;
+  display: flex;
+  flex-direction: column;
+}
+
 .terminal-container {
   flex: 1;
   min-width: 0;
@@ -639,6 +647,59 @@ body {
   padding: 0 4px;  /* keep text off the side edges */
 }
 
+/* Terminal search bar */
+.terminal-search-bar {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  padding: 4px 8px;
+  background: var(--bg-surface);
+  border-bottom: 1px solid var(--border);
+  flex-shrink: 0;
+}
+
+.terminal-search-bar.hidden {
+  display: none;
+}
+
+.terminal-search-input {
+  flex: 1;
+  background: var(--bg);
+  color: var(--text);
+  border: 1px solid var(--border);
+  border-radius: 4px;
+  padding: 4px 8px;
+  font-family: var(--font-ui);
+  font-size: 13px;
+  outline: none;
+}
+
+.terminal-search-input:focus {
+  border-color: var(--accent);
+}
+
+.terminal-search-count {
+  color: var(--text-muted);
+  font-size: 12px;
+  min-width: 40px;
+  text-align: center;
+}
+
+.terminal-search-btn {
+  background: none;
+  border: 1px solid var(--border);
+  border-radius: 4px;
+  color: var(--text-muted);
+  cursor: pointer;
+  padding: 2px 8px;
+  font-size: 12px;
+}
+
+.terminal-search-btn:hover {
+  color: var(--text);
+  border-color: var(--accent);
+}
+
 /* xterm.js injects overflow-y: scroll on .xterm-viewport — override it */
 .xterm .xterm-viewport {
   overflow-y: hidden !important;