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/config/config.go

@@ -10,16 +10,18 @@ import (
 
 // Config is the validated runtime configuration. Fields are ordered
 // largest-alignment-first to satisfy the fieldalignment linter.
+//
+// Temperature is intentionally not configurable: captcha solving is fixed at
+// temperature 0 for deterministic results (see the LLM provider request bodies).
 type Config struct {
-	Provider    string
-	BaseURL     string
-	APIKey      string
-	Model       string
-	AudioModel  string
-	LogLevel    string
-	Temperature float64
-	Timeout     time.Duration
-	MaxTokens   int
+	Provider   string
+	BaseURL    string
+	APIKey     string
+	Model      string
+	AudioModel string
+	LogLevel   string
+	Timeout    time.Duration
+	MaxTokens  int
 }
 
 var validProviders = map[string]bool{
@@ -47,13 +49,6 @@ func Load() (Config, error) {
 	if c.Model == "" {
 		return Config{}, fmt.Errorf("CAPTCHA_LLM_MODEL is required")
 	}
-	if v := os.Getenv("CAPTCHA_LLM_TEMPERATURE"); v != "" {
-		f, err := strconv.ParseFloat(v, 64)
-		if err != nil {
-			return Config{}, fmt.Errorf("CAPTCHA_LLM_TEMPERATURE: %w", err)
-		}
-		c.Temperature = f
-	}
 	if v := os.Getenv("CAPTCHA_LLM_MAX_TOKENS"); v != "" {
 		n, err := strconv.Atoi(v)
 		if err != nil {