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/json_test.go

@@ -0,0 +1,38 @@
+package captcha
+
+import "testing"
+
+func TestExtractJSON(t *testing.T) {
+	tests := []struct {
+		name string
+		in   string
+		want string
+	}{
+		{"plain", `{"text":"ab12"}`, `{"text":"ab12"}`},
+		{"prose wrapped", `Sure, here is the answer: {"text":"ab12"} hope it helps`, `{"text":"ab12"}`},
+		{
+			"markdown fenced",
+			"```json\n{\"tiles\":[1,2,3]}\n```",
+			`{"tiles":[1,2,3]}`,
+		},
+		{
+			"nested object",
+			`prefix {"a":{"b":1},"c":2} suffix`,
+			`{"a":{"b":1},"c":2}`,
+		},
+		{
+			"first balanced object wins",
+			`For example {"text":"EXAMPLE"} but the real one is {"text":"REAL"}`,
+			`{"text":"EXAMPLE"}`,
+		},
+		{"no brace", `no json here`, `no json here`},
+		{"unbalanced returns from start", `tail {"text":"x"`, `{"text":"x"`},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := extractJSON(tc.in); got != tc.want {
+				t.Fatalf("got %q want %q", got, tc.want)
+			}
+		})
+	}
+}