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/css/chat.css

@@ -159,6 +159,29 @@
   background: var(--f2-bg);
 }
 
+/* Steering: Live-Einwurf während eine Chain läuft */
+.msg.user.steer {
+  border-color: var(--warning);
+  border-left-width: 3px;
+}
+
+.steer-label {
+  font-size: .72em;
+  font-weight: 600;
+  color: var(--warning);
+  text-transform: uppercase;
+  letter-spacing: .04em;
+  margin-bottom: 4px;
+  opacity: .9;
+}
+
+.steer-label .fa { margin-right: 3px; }
+
+/* Eingabefeld signalisiert "Einwurf-Modus" solange ein Stream läuft */
+#input.steer-mode {
+  box-shadow: inset 0 0 0 1px var(--warning);
+}
+
 .msg.assistant {
   align-self: flex-start;
   background: var(--f1-bg);