feat: context window usage indicator with real agent data

The context indicator in the composer footer now shows real data from
the agent's context compressor instead of hardcoded estimates:

- last_prompt_tokens / context_length (e.g. '12.4k / 200k (6%)')
- Bar color: blue <50%, yellow 50-75%, red >75%
- Hover tooltip shows exact numbers + compression threshold
- Cost appended when available

Backend: streaming.py now reads context_length, threshold_tokens, and
last_prompt_tokens from agent.context_compressor after run_conversation()
and includes them in the usage dict sent with the 'done' SSE event.

This matches the CLI's context window display (the bar that shows
current context vs total window).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/streaming.py

@@ -310,6 +310,12 @@ def _run_agent_streaming(session_id, msg_text, model, workspace, stream_id, atta
                             break
             s.save()
             usage = {'input_tokens': input_tokens, 'output_tokens': output_tokens, 'estimated_cost': estimated_cost}
+            # Include context window data from the agent's compressor for the UI indicator
+            _cc = getattr(agent, 'context_compressor', None)
+            if _cc:
+                usage['context_length'] = getattr(_cc, 'context_length', 0) or 0
+                usage['threshold_tokens'] = getattr(_cc, 'threshold_tokens', 0) or 0
+                usage['last_prompt_tokens'] = getattr(_cc, 'last_prompt_tokens', 0) or 0
             put('done', {'session': s.compact() | {'messages': s.messages, 'tool_calls': tool_calls}, 'usage': usage})
           finally:
             if old_cwd is None: os.environ.pop('TERMINAL_CWD', None)