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/lib/layout/LeafPane.css

@@ -61,11 +61,11 @@
   height: 24px;
   box-sizing: border-box;
   /* Lock height: a narrow pane used to wrap toolbar items to 2+ rows,
-     which shrank the xterm beneath and reflowed the terminal. Clip
-     overflow instead of showing a scrollbar (which would itself eat
-     into the 24px and crush the buttons). */
+     which shrank the xterm beneath and reflowed the terminal. nowrap +
+     flex-shrink:0 keeps items at natural width on one row; overflow is
+     left visible so the shell-picker dropdown (rendered below the
+     toolbar) isn't clipped. */
   white-space: nowrap;
-  overflow: hidden;
 }
 .pane-toolbar > * {
   flex-shrink: 0;