Show Qwen3 thinking output in chat as collapsible Reasoning block

Ollama streams message.thinking tokens alongside message.content when
think=True — previously silently dropped. Now forwarded end-to-end.

Backend:
- llm.py: ChatChunk type gains "thinking" variant; stream_chat_with_tools
  yields ChatChunk(type="thinking") for msg.thinking chunks before content
- generation_task.py: thinking chunks emit "thinking_chunk" SSE events
  (not added to content_so_far — not persisted to DB)

Frontend:
- types/chat.ts: Message.thinking?: string (session-only, not from DB)
- stores/chat.ts: streamingThinking ref; thinking_chunk handler accumulates
  chunks; on done, thinking carried into committed Message object then cleared
- ChatMessage.vue: collapsible <details class="thinking-block"> shown for
  messages that have .thinking content (collapsed by default)
- ChatView.vue + ChatPanel.vue: live thinking block in streaming bubble —
  open while only thinking is flowing, auto-collapses when content arrives;
  typing indicator hidden while thinking is active

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/services/generation_task.py

@@ -442,7 +442,10 @@ async def run_generation(
                     cancelled = True
                     break
 
-                if chunk.type == "content":
+                if chunk.type == "thinking":
+                    buf.append_event("thinking_chunk", {"chunk": chunk.content})
+
+                elif chunk.type == "content":
                     if timing["ttft_ms"] is None:
                         timing["ttft_ms"] = int((time.monotonic() - t_start) * 1000)
                     buf.content_so_far += chunk.content