feat(run): Enter sends the prompt, and a busy lane attaches instead of failing

- The prompt box sends on Enter (Ctrl/Cmd+Enter still works); Shift+Enter
  inserts a newline, and IME composition (Vietnamese/CJK) never sends.
- Starting or resuming into a lane that still holds a live run answered a
  raw 409 ERUNLIVE; the console now attaches to that run with a notice.
  Checked by error code, since the server's message is prose.
- Active runs gains a two-click Kill for live dashboard-spawned rows, the
  way to free a lane when the previous session really should go.

client/src/components/run/RunSetup.tsx

@@ -77,7 +77,9 @@ interface PromptEditorProps {
 
 function PromptEditor({ value, onChange, onSubmit, placeholder, rows = 5 }: PromptEditorProps) {
   const handleKeyDown = (e: React.KeyboardEvent<HTMLTextAreaElement>) => {
-    if ((e.ctrlKey || e.metaKey) && e.key === "Enter") {
+    // Enter sends (Ctrl/Cmd+Enter still does); Shift+Enter inserts a newline.
+    // isComposing guards IME input (Vietnamese/CJK), where Enter commits the candidate.
+    if (e.key === "Enter" && !e.shiftKey && !e.nativeEvent.isComposing) {
       e.preventDefault();
       onSubmit();
     }