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/stealth/interaction.go

@@ -7,7 +7,7 @@ import (
 
 // Action is a high-level target the caller wants to perform.
 type Action struct {
-	Type string `json:"type"` // click | type | scroll
+	Type string `json:"type"` // click | type
 	Text string `json:"text,omitempty"`
 	X    int    `json:"x,omitempty"`
 	Y    int    `json:"y,omitempty"`