feat(web): expose continuable subagent interrupt

The composer's single primary action now stops a running continuable
child: Session.cancel() routes a continuable address through
subagent.interrupt (ordinary sessions keep session.cancel; one-shot
addresses stay uncancellable), and InputBar switches the same Send button
to Stop while a continuable child runs. A running parent-offline child
keeps the default composer with disabled input so that Stop stays
reachable; the read-only takeover returns once it stops.

Refs #1535

apps/web/tests/snapshots/subagent-interrupt/offline-composer.expected.md

@@ -0,0 +1,25 @@
+- banner:
+  - navigation "Session hierarchy":
+    - button "Ask a research subagent to"
+    - text: /
+    - button "event-sourcing researcher" [disabled]
+  - tablist:
+    - tab "Chat" [selected]
+    - tab "Trajectory"
+- text: Explain event sourcing in one sentence. {{clock}}
+- button "Copy":
+  - img
+- button "Branch into a new conversation" [disabled]:
+  - img
+- text: Available only on the last message of a completed turn
+- button "Context injection @deepseek-ai/dsh-system-prompt":
+  - img
+  - img
+  - text: Context injection @deepseek-ai/dsh-system-prompt
+- paragraph: partial
+- status: Deep diving...
+- textbox "Parent session offline; sending is unavailable but you can still stop the run" [disabled]
+- button "Commands" [disabled]:
+  - img
+- 'button "Access mode, current: Workspace Write" [disabled]': Workspace Write
+- button "Stop generating"