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/chat.js

@@ -136,6 +136,12 @@ function handleStreamEvent(eventType, data) {
       addWarningBubble(`Auto-Continue (${data.attempt}/${data.maxAttempts}): Modell wird fortgesetzt…`, true);
       break;
 
+    case 'steer_injected':
+      // Das Modell hat den Einwurf an dieser Stelle erhalten. Die Anzeige übernimmt
+      // bereits die optimistische Bubble aus sendSteer(); am Stream-Ende ersetzt
+      // pollHistory() sie durch die persistierte DB-Version (Type=steer).
+      break;
+
     case 'warning':
     case 'info':
       addWarningBubble(data.message, true);
@@ -278,6 +284,14 @@ async function _runStream(fetchPromise) {
   _stopClickCount = 0;
   startTypingIndicator();
 
+  // Während des Streams wird die Eingabe zum Steering-Einwurf (Enter wirft ein).
+  const _steerInputEl = $('#input');
+  const _prevPlaceholder = _steerInputEl ? _steerInputEl.placeholder : '';
+  if (_steerInputEl) {
+    _steerInputEl.placeholder = '💬 Einwurf an die laufende Arbeit… (Enter)';
+    _steerInputEl.classList.add('steer-mode');
+  }
+
   try {
     const resp = await fetchPromise(_abortController.signal);
     const reader = resp.body.getReader();
@@ -331,6 +345,11 @@ async function _runStream(fetchPromise) {
       stopBtn.title = 'Antwort abbrechen';
       isStreaming = false;
       _abortController = null;
+      // Steering-Hinweis zurücksetzen (Eingabe ist wieder normales Senden)
+      if (_steerInputEl) {
+        _steerInputEl.placeholder = _prevPlaceholder;
+        _steerInputEl.classList.remove('steer-mode');
+      }
       // Meta zurücksetzen falls kein done-Event kam (z.B. nach Stop)
       var metaEl = $('#meta');
       if (metaEl && (metaEl.textContent.includes('Streaming') || metaEl.textContent.includes('...'))) {
@@ -350,8 +369,14 @@ async function _runStream(fetchPromise) {
 // ── Send Message ──
 
 async function sendMessage() {
-  if (isStreaming) return;
   const input = $('#input');
+  if (isStreaming) {
+    // Steering: Während eine Chain läuft, wird die Nachricht als Einwurf in die
+    // laufende Arbeit eingespeist (statt wie früher hart blockiert zu werden).
+    const steerText = input.value.trim();
+    if (steerText) sendSteer(steerText);
+    return;
+  }
   const text = input.value.trim();
   if (!text && pendingFiles.length === 0 && pendingRefs.length === 0) return;
 
@@ -424,6 +449,34 @@ async function sendMessage() {
   await _runStream((signal) => apiChat(content, getCurrentSessionId(), signal, _planModeActive));
 }
 
+// ── Steering (Einwurf in die laufende Chain) ──
+async function sendSteer(text) {
+  const input = $('#input');
+  // Eingabe leeren + optimistische Bubble (wird am Stream-Ende durch die DB-Version ersetzt)
+  input.value = '';
+  input.dispatchEvent(new Event('input'));
+  const bubble = addSteerBubble(text, true);
+  scrollDownForce();
+
+  let res;
+  try {
+    res = await apiChatSteer(getCurrentSessionId(), text);
+  } catch (e) {
+    if (bubble) bubble.remove();
+    input.value = text;
+    input.dispatchEvent(new Event('input'));
+    addWarningBubble('Einwurf fehlgeschlagen: ' + (e.message || e));
+    return;
+  }
+  if (!res || !res.injected) {
+    // Race: die laufende Antwort ist gerade fertig geworden → Text zurücklegen, normal erneut senden
+    if (bubble) bubble.remove();
+    input.value = text;
+    input.dispatchEvent(new Event('input'));
+    addWarningBubble('Die laufende Antwort ist bereits beendet – bitte erneut senden.');
+  }
+}
+
 // ── Direct Message (für Inline-Choice-Buttons) ──
 async function sendMessageDirect(text) {
   if (isStreaming || !text) return;