fix(studio): improve chat context, thinking tags, streaming, and tool results

- Fix cleanThinkingTags to use proper regex instead of naive ReplaceAll
- Send conversation history (last 20 messages + summary) to AI instead of single message
- Store tool results alongside tool calls so history shows complete execution info
- Stream words instead of characters for smoother SSE rendering
- Add stop button to cancel in-progress AI requests (AbortController)
- Fix markdown rendering: add h2 support, use div for bullets
- Add i18n keys for cancel/stop (EN + FR)

💘 Generated with Crush

Assisted-by: GLM-5.1 via Crush <crush@charm.land>

internal/api/handlers_shell_chat.go

@@ -136,8 +136,13 @@ func (s *Server) handleShellChatStream(w http.ResponseWriter, orb *orchestrator.
 		content := cleanThinkingTags(choice.Message.Content)
 
 		if content != "" {
-			for _, ch := range strings.Split(content, "") {
-				writeSSE(map[string]interface{}{"content": ch})
+			words := strings.Fields(content)
+			for i, w := range words {
+				chunk := w
+				if i < len(words)-1 {
+					chunk += " "
+				}
+				writeSSE(map[string]interface{}{"content": chunk})
 			}
 			finalContent = content
 		}