add status-bar collapse toggles for sidebar + context pane (T-294)

Fixed ~24px caret-line cells bookend the status bar (left=sidebar, right=context);
chevron flips per arrangement.isCollapsed (inward=collapse, outward=expand) and
fires the existing sidebar.collapse / context.collapse commands — the mouse
affordance for the already keyboard/CLI-addressable action (D-6). Visual only;
no new collapse logic. Status items shift 28px inward for the reserved cells.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -18,6 +18,12 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Added
 
+- **Collapse toggles in the status bar.** A small caret-line button bookends
+  each end of the bottom status bar — left collapses/expands the sidebar, right
+  the context pane. The chevron points inward to collapse, outward to expand,
+  and fires the existing `sidebar.collapse` / `context.collapse` commands
+  (`Ctrl+Shift+1` / `Ctrl+Shift+3`), so it's the mouse affordance for an
+  already keyboard/CLI-addressable action. (T-294)
 - **Pasted images render inline in the Claude conversation.** A pasted-image
   `@<path>` reference now shows as a bounded thumbnail in the message instead of
   the raw path; clicking it (or pressing Enter when focused) opens the full