fix: address code review (v3/v2 detect, gemini caps, fetch timeout, audio mime, json extract, dead temp config)

- detect: recaptcha_v3 now suppresses recaptcha_v2 (hidden g-recaptcha div on v3 pages no longer mis-detected as LLM-solvable); funcaptcha drops over-broad bare "arkose" substring
- llm/gemini: stub methods return ErrUnsupported{Provider:"gemini"} so audio handler errors.As guard catches it; drop unused fmt import
- mcp/tools: remove gemini from audio capability (stub); pick audio filename (.wav/.mp3) from sniffed media type; schema notes for browser/actions no longer advertise unimplemented options
- imageutil: fetchURL uses a package client with 30s Timeout instead of http.DefaultClient
- captcha/json: extractJSON brace-counts to return the first balanced object (handles prose, markdown fences, example-before-real)
- config: remove dead CAPTCHA_LLM_TEMPERATURE / Config.Temperature; solving is fixed at temperature 0
- stealth: Action.Type doc comment drops nonexistent scroll case

internal/imageutil/imageutil.go

@@ -16,6 +16,8 @@ import (
 
 var dataURIRe = regexp.MustCompile(`^data:([^;,]+)(;base64)?,(.*)$`)
 
+var fetchClient = &http.Client{Timeout: 30 * time.Second}
+
 // Point is a pixel coordinate.
 type Point struct {
 	X int
@@ -65,7 +67,7 @@ func fetchURL(url string) ([]byte, string, error) {
 	if err != nil {
 		return nil, "", err
 	}
-	resp, err := http.DefaultClient.Do(req)
+	resp, err := fetchClient.Do(req)
 	if err != nil {
 		return nil, "", err
 	}