fix(studio): forward AI thinking chunks to frontend instead of dropping them

The ThinkingBlock component existed but was dead code — the backend
silently discarded all <think chunks. Now emits thinking SSE events
so the UI can display AI reflections in real-time.

\xe2\x98\x85 Generated with Crush

Assisted-by: GLM-5-Turbo via Crush <crush@charm.land>

web/src/api/client.js

@@ -68,7 +68,9 @@ const api = {
               if (data.done) { resolve(full); return }
               if (data.content) {
                 full += data.content
-                if (onChunk) onChunk(full)
+                if (onChunk) onChunk(full, data)
+              } else if (data.thinking !== undefined || data.thinking_end) {
+                if (onChunk) onChunk(full, data)
               }
             } catch {}
           }