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/pages/__tests__/__snapshots__/screens.snapshot.test.tsx.snap

@@ -5578,7 +5578,7 @@ exports[`screen snapshots > Run 1`] = `
                   <div
                     class="mt-1 text-[10px] text-fg-muted"
                   >
-                    Cmd+Enter / Ctrl+Enter to send
+                    Enter to send · Shift+Enter for newline
                      · / for slash commands · @ for file references
                   </div>
                 </div>
@@ -9617,7 +9617,7 @@ exports[`screen snapshots > Settings 1`] = `
             <p
               class="text-[10px] text-status-warning/90 mt-1"
             >
-              UI build v1.8.1 (rebuild client to match)
+              UI build v1.8.2 (rebuild client to match)
             </p>
           </div>
           <div