Orbit: brain restart doesn't reset renderer UI (badge stuck on "thinking…", Stop button stays visible)

## Symptom

Forcing a brain restart via \`/resume\`, the Restart Agent menu item, or the implicit prefs-save restart leaves the renderer in a stuck \"streaming\" UI state:

- Status badge stays on \"thinking…\".
- Stop button stays visible; Send hidden.
- Input field accepts text but submits get queued instead of fired.
- \`streaming === true\` never flips back to false.

User has to **Ctrl+R** the window to escape.

## Why

Renderer's \`streaming\` flag flips to false only on the brain's \`agent_end\` event (\`app.ts\` chat-stream handler). \`agent.stop()\` in main kills the brain process; the brain never gets a chance to emit \`agent_end\`; the renderer waits forever.

## Proposed fix

\`AgentManager.stop()\` already calls \`setStatus(\"stopped\")\`, which the renderer receives via \`agent:status\`. Wire the renderer's \`onAgentStatus\` handler to also reset turn state:

\`\`\`ts
window.orbit.onAgentStatus((status, msg) => {
setStatusBadge(status, msg);
if (status === \"stopped\" || status === \"error\") {
streaming = false;
sendBtn.classList.remove(\"hidden\");
abortBtn.classList.add(\"hidden\");
// any other turn-end cleanup
}
...
});
\`\`\`

Or factor the turn-end cleanup into \`resetUiForFreshContext\` and call it on stopped/error transitions. Either works.

## Files

- \`app/src/renderer/app.ts\` — \`onAgentStatus\` handler (~line 1918) + a small reset helper.

## Repro

1. Send a slow prompt.
2. Type \`/resume\` while it's streaming.
3. Watch: brain restarts cleanly per \`/tmp/orbit.log\` but UI stays stuck on \"thinking…\".

## Related

- #62 (prefs-save kills in-flight prompt) — same root: brain lifecycle events don't unwind renderer state cleanly.