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/llm/gemini.go

@@ -2,7 +2,6 @@ package llm
 
 import (
 	"context"
-	"fmt"
 	"net/http"
 )
 
@@ -16,9 +15,9 @@ type geminiClient struct {
 }
 
 func (c *geminiClient) Vision(_ context.Context, _ string, _ []Image, _ Options) (string, error) {
-	return "", fmt.Errorf("gemini provider not yet implemented")
+	return "", ErrUnsupported{Provider: "gemini"}
 }
 
 func (c *geminiClient) Transcribe(_ context.Context, _ []byte, _ string, _ Options) (string, error) {
-	return "", fmt.Errorf("gemini provider not yet implemented")
+	return "", ErrUnsupported{Provider: "gemini"}
 }