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/bubbles.js

@@ -157,6 +157,27 @@ function addUserBubble(text, streamed = false, timestamp = '', messageId = null,
   scrollDown();
 }
 
+// ── Steering Bubble (Live-Einwurf während eine Chain läuft) ──
+
+/** Baut eine Steering-Bubble (User-Stil + Einwurf-Label). Returns den div – für Live + History. */
+function _buildSteerBubble(opts) {
+  const div = _createMessageBubble({ role: 'user', extraClass: 'user steer', timestamp: opts.timestamp, messageId: opts.messageId, isMarked: opts.isMarked, deletedOn: opts.deletedOn, archived: opts.archived, streamed: opts.streamed });
+  const label = document.createElement('div');
+  label.className = 'steer-label';
+  label.innerHTML = '<i class="fa fa-bolt"></i> Einwurf';
+  div.appendChild(label);
+  renderUserContent(div, opts.text);
+  _addBubbleActions(div, { timestamp: opts.timestamp, messageId: opts.messageId, deletedOn: opts.deletedOn, isMarked: opts.isMarked });
+  return div;
+}
+
+function addSteerBubble(text, streamed = false) {
+  const div = _buildSteerBubble({ text, streamed });
+  chatEl().appendChild(div);
+  scrollDown();
+  return div;
+}
+
 function _addSystemBubble(text, icon, color, streamed) {
   const div = document.createElement('div');
   div.className = 'msg system-info';