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>

wwwroot/js/history.js

@@ -261,6 +261,20 @@ function _renderMessagesIntoFragment(fragment, msgs, imageMessageIds, opts) {
         }
         break;
       }
+      case 'steer': {
+        // Live-Einwurf des Users während die Chain lief (User-Stil + Einwurf-Label)
+        if (msg.role === 'user' && msg.content) {
+          fragment.appendChild(_buildSteerBubble({
+            text: msg.content,
+            timestamp: msg.createdAt,
+            messageId: msg.id,
+            isMarked: msg.isMarked,
+            deletedOn: msg.deletedOn,
+            archived: archived
+          }));
+        }
+        break;
+      }
       case 'auto_context': {
         if (msg.content) {
           const acDiv = document.createElement('div');