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/captcha/detect_test.go

@@ -13,7 +13,7 @@ func TestDetectHTML(t *testing.T) {
 		{"recaptcha v3", `<script src="https://www.google.com/recaptcha/api.js?render=6Lc_v3key"></script>`, "recaptcha_v3", false},
 		{"hcaptcha", `<div class="h-captcha" data-sitekey="hk_123"></div>`, "hcaptcha", true},
 		{"turnstile", `<div class="cf-turnstile" data-sitekey="0x4AAA"></div>`, "turnstile", false},
-		{"funcaptcha", `<div id="arkose" data-pkey="ABC"></div>`, "funcaptcha", true},
+		{"funcaptcha", `<div class="arkoselabs" data-pkey="ABC"></div>`, "funcaptcha", true},
 	}
 	for _, tc := range tests {
 		t.Run(tc.name, func(t *testing.T) {
@@ -27,3 +27,19 @@ func TestDetectHTML(t *testing.T) {
 		})
 	}
 }
+
+// A v3 page often also carries a hidden g-recaptcha div; only v3 must fire.
+func TestDetectHTMLV3SuppressesV2(t *testing.T) {
+	html := `<script src="https://www.google.com/recaptcha/api.js?render=6Lc_v3key"></script>` +
+		`<div class="g-recaptcha" data-sitekey="6Lc_hidden" style="display:none"></div>`
+	got := DetectHTML(html)
+	if len(got) != 1 {
+		t.Fatalf("want exactly 1 detection, got %d: %+v", len(got), got)
+	}
+	if got[0].Type != "recaptcha_v3" {
+		t.Fatalf("want recaptcha_v3, got %q", got[0].Type)
+	}
+	if got[0].LLMSolvable {
+		t.Fatalf("recaptcha_v3 must not be LLMSolvable")
+	}
+}