refactor(toolbar): one instance stepper, not two

The desktop toolbar carried two identical "minus 1 plus" instance steppers side
by side, one after Run and one after Run Shell. The second (#shellCount) is
gone for a cleaner strip.

Run Shell keeps the capability: both launch paths now read the remaining
#tabCount control through _toolbarInstanceCount(), which also makes an absent
stepper read as 1 instead of throwing. That matters because the group is
display:none on phones and tablets, and because the Run dropdown's
Terminal / Shell entry routes through runShell() too, where the visible counter
was previously ignored.

Desktop only: both steppers were already hidden under 1024px.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/wiki/The-Dashboard.md

@@ -15,7 +15,7 @@ page says so and names the setting.
 | **Header, left**   | The "C" logo (goes home) and the session list, unless you moved it to the sidebar.       |
 | **Header, right**  | Status chips and panel buttons, most of them off by default.                            |
 | **Center**         | The terminal for the active session, or the home screen when nothing is selected.        |
-| **Bottom toolbar** | Run, Stop, Run Shell, the case picker, and the instance counters.                        |
+| **Bottom toolbar** | Run, Stop, Run Shell, the case picker, and the instance counter.                         |
 | **Overlays**       | Panels and modals: Respawn, Cron, Subagents, File Viewer, Settings.                      |
 
 ## Session list layout