feat(toolbar): move "+" and the case gear into the case picker

The owner asked to drop the "+" (Add Case) and gear (case settings) buttons
beside the toolbar case picker. Their two actions now close the picker's own
list instead, as "New or link a case…" and "Case settings…" rows in a sticky
footer, so nothing becomes unreachable: on desktop the "+" was the only door to
Add Case (create, link, clone, manage), and the gear the only one to the
per-case Agent Teams / 1M Opus overrides.

The rows are part of the arrow-key walk (after the last case) and Enter runs
them, they also show when nothing matches the filter, and the case settings
popover still opens anchored to the case group. The picker input gets its
right-hand corners back, and the dead .btn-case-add / .btn-case-settings rules
(desktop, phone and two skin selector lists) are gone. Phones keep their own
case sheet and gear.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/wiki/Core-Concepts.md

@@ -15,7 +15,8 @@ Codeman-side configuration:
 - Per-case toggles (Agent Teams, 1M Opus context).
 - Where it runs, if it is not the local filesystem: see [Location overlays](#location-overlays).
 
-Three ways to get one, all under **+** next to the case picker:
+Three ways to get one, all under **New or link a case…** at the bottom of the case picker
+(the case dropdown in the bottom toolbar; on a phone, the case sheet's **Create New Case**):
 
 | How               | Result                                                                                                 |
 | ----------------- | ------------------------------------------------------------------------------------------------------ |