feat(maestro): force-kill prompt in the web client

The kill button now sends the unforced DELETE immediately instead of
asking first. Only the daemon's guard refusal raises a prompt, and it is
a real dialog rather than confirm(), so it can offer the override the
daemon already implements.

The API error envelope is decoded into ApiRequestError, keeping the code
and the unsaved-work breakdown so the dialog names what is in the way.

docs/WORKTREES.md

@@ -79,6 +79,11 @@ would have blocked. A tree whose guard is clear never needed the override, so a
 forced kill removes it as usual — force changes what happens to blocked work,
 not ordinary cleanup.
 
+In the web client the kill button sends the unforced `DELETE` straight away —
+there is no pre-confirmation, because the daemon is the one that decides. Only
+the refusal raises a prompt, and that prompt names what is in the way and offers
+"Force kill", which repeats the request with `force=true`.
+
 Ending a session and destroying its work are two separate acts. Maestro only
 performs the first one. Removing a tree it left behind is the operator's own
 `git worktree remove`, chosen while looking at exactly what would be lost.