Add live steering injection into running chains

Let the user inject a message into a running chain instead of only being
able to abort it. While a chain streams, typing + Enter sends the text to
a new /api/chat/steer endpoint; it is drained at the next iteration
boundary, wrapped with a hint so the model actively classifies it
("complements the task -> keep working" vs. "changes course -> adjust"),
and stored as a Type="steer" user message so it survives reload.

Shared infrastructure (provider-agnostic):
- SteeringQueue: out-of-band queue keyed by sessionId, mirrored from
  StreamingTracker (Enqueue/Drain/Clear).
- Program.cs registers it as a singleton and passes it to AnthropicService.
- ChatEndpoint: POST /api/chat/steer enqueues only while IsStreaming,
  Clear() in the finally.
- ChatService.BuildHistoryFromDb groups Type="steer" so no two
  consecutive user turns are produced on re-chat.
- Frontend api.js/chat.js/bubbles.js/history.js + chat.css render the
  optimistic steer bubble and the steer_injected SSE event; Enter during
  a stream sends a steer instead of being blocked.

Anthropic API path:
- Drain point B (tool boundary, merged into the tool_result user turn)
  and C (turn end -> inject + continue instead of finalize).

Claude CLI path (kill-and-restart):
- The agentic loop runs inside the claude child, so true mid-turn inject
  is impossible. On a pending steer the child is killed at the next tool
  boundary, progress is rebuilt from the DB as a <work_in_progress>
  transcript, and the CLI is restarted with the steer appended.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Program.cs

@@ -52,6 +52,8 @@ builder.Services.AddSingleton(authService);
 builder.Services.AddSingleton(dbService);
 builder.Services.AddSingleton(new ImageService());
 builder.Services.AddSingleton(new StreamingTracker());
+var steeringQueue = new SteeringQueue();
+builder.Services.AddSingleton(steeringQueue);
 builder.Services.AddSingleton(new RemoteConnectionManager());
 var memoryService = new MemoryService(dataDir);
 var skillService = new SkillService(dataDir);
@@ -88,7 +90,8 @@ builder.Services.AddSingleton<AnthropicService>(sp =>
         toolService, dbService, memoryService,
         serverSettings,
         vectorSearch: vectorSearchService,
-        embeddingBg: embeddingBgService);
+        embeddingBg: embeddingBgService,
+        steeringQueue: steeringQueue);
     svc.SetChatService(chatService);
     return svc;
 });