feat(onboarding): add minimax api key step and AI-powered editor scan

- Add apikey step in onboarding wizard (optional, with validation)
- Add ScanEditors() in scanner package detecting vim/nvim/code/emacs/nano/helix/subl/zed
- Add GET /api/editors endpoint
- Editor step now has scan button to detect installed editors via backend
- MiniMax API key is saved to provider config if provided

💘 Generated with Crush

Assisted-by: MiniMax-M2.7 via Crush <crush@charm.land>

internal/api/handlers_info.go

@@ -117,3 +117,8 @@ func (s *Server) handleScan(w http.ResponseWriter, r *http.Request) {
 	s.scanResult = scanner.ScanSystem()
 	writeJSON(w, map[string]string{"status": "ok"})
 }
+
+func (s *Server) handleEditors(w http.ResponseWriter, r *http.Request) {
+	editors := scanner.ScanEditors()
+	writeJSON(w, map[string]interface{}{"editors": editors})
+}