Tidy titlebar: dropdowns for shell + layout, '+' button to spawn

- Collapse the inline distro buttons + PowerShell + 🔑 SSH hosts into
  a single 'Ubuntu ▾' dropdown (WSL distros + PowerShell sections),
  with 🔑 as a separate icon-only button.
- Collapse the 5 preset buttons into a 'layout ▾' dropdown.
- Add a '+' button next to the shell picker that spawns a new pane of
  the picked shell by splitting the active pane (smart orientation:
  splits right if wide, down if tall). Per-pane ⇥/⇣ arrows still
  inherit from parent — only '+' uses the titlebar selection.
- Drop the 🔔 test-toast button.
- Drop overflow:hidden from titlebar + pane toolbar so dropdowns
  aren't clipped; height lock + nowrap still prevent the reflow bug.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/App.css

@@ -20,12 +20,11 @@
      text would otherwise wrap (e.g. "📡 all off") would grow the
      titlebar, shrink .pane-wrap, and reflow every xterm. nowrap stops
      text-wrap inside buttons, flex-shrink:0 stops children from being
-     squeezed, height locks the row height, overflow:hidden clips items
-     that don't fit (widen the window to see them). */
+     squeezed, height locks the row height. Overflow is left visible
+     so dropdown menus below the chips aren't clipped by the bar. */
   white-space: nowrap;
   height: 34px;
   box-sizing: border-box;
-  overflow: hidden;
 }
 .titlebar > * {
   flex-shrink: 0;
@@ -35,12 +34,7 @@
   color: #ddd;
 }
 
-.distros, .presets {
-  display: flex;
-  gap: 4px;
-  align-items: center;
-}
-.distro-btn, .preset-btn, .palette-btn {
+.titlebar-chip, .palette-btn {
   font: inherit;
   font-family: "Cascadia Mono", "JetBrains Mono", "Consolas", monospace;
   font-size: 11px;
@@ -51,15 +45,21 @@
   padding: 2px 8px;
   cursor: pointer;
 }
-.distro-btn:hover, .preset-btn:hover, .palette-btn:hover {
+.titlebar-chip:hover, .palette-btn:hover {
   background: #2a2a2a;
   color: #ddd;
 }
-.distro-btn.active {
-  background: #1a3a5c;
+.titlebar-chip.add-pane {
+  font-size: 14px;
+  line-height: 1;
+  padding: 2px 8px;
   color: #cce6ff;
   border-color: #2a5a8c;
 }
+.titlebar-chip.add-pane:hover {
+  background: #1a3a5c;
+  color: #fff;
+}
 .palette-btn.bcast-all.on {
   background: #4a3010;
   color: #f0c060;
@@ -74,14 +74,6 @@
   color: #80e080;
   border-color: #2a6a2a;
 }
-.preset-btn {
-  min-width: 28px;
-  text-align: center;
-}
-.muted {
-  color: #666;
-  font-style: italic;
-}
 .layout-info {
   margin-left: auto;
   font-family: "Cascadia Mono", "JetBrains Mono", "Consolas", monospace;