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/i18n/locales/en/run.json

@@ -34,6 +34,7 @@
   },
   "errors": {
     "attachFailed": "Couldn't attach to run: {{message}}",
+    "attachedToLiveInstead": "This lane's previous session is still running, so nothing new could start — attached to it instead. Kill it from Active runs to start fresh.",
     "cwdRequired": "Working directory required to start a run",
     "killFailed": "Couldn't stop: {{message}}",
     "laneCreateFailed": "Failed to create lane: {{message}}",
@@ -97,7 +98,7 @@
     "headlessExplain": "Headless mode is best for scripted tasks where you know exactly what you want. The session can't ask follow-up questions and will hang on permission prompts unless you stay in acceptEdits.",
     "permissionWarning": "bypassPermissions allows ALL tool invocations without confirming. Only use for trusted prompts.",
     "sandboxWarning": "Codex will run commands with no sandbox.",
-    "shortcut": "Cmd+Enter / Ctrl+Enter to send"
+    "shortcut": "Enter to send · Shift+Enter for newline"
   },
   "limitations": {
     "dismiss": "Got it"