fetch: every outbound request carries one generic browser User-Agent

Stripping forwarding headers kept our IP and chain out of reach, but the
User-Agent went through untouched. Clients that fell back to their own name
("crawlkit-roles", "crawlkit-virk", ...) tied all ten exits together as one
actor, and one missing UA meant Go-http-client. The proxy now overwrites the
UA on every request (WEIRCON_USER_AGENT, default current Firefox) and the
egress prober uses the same string.

Also dropped: From, Sec-Ch-Ua* (would contradict a Firefox UA) and any
X-Weircon-* by prefix. New end-to-end test drives the whole handler and
checks what the target actually receives.

Claude-Session: https://claude.ai/code/session_01AUj1LSMmrowcomkn1ciAgj

service/main_test.go

@@ -2,7 +2,10 @@ package main
 
 import (
 	"net/http"
+	"net/http/httptest"
+	"strings"
 	"testing"
+	"time"
 )
 
 func TestStripIdentifying(t *testing.T) {
@@ -25,6 +28,10 @@ func TestStripIdentifying(t *testing.T) {
 		"Fastly-Client-Ip",
 		"X-Original-Forwarded-For",
 		"X-Client-Ip",
+		"X-Weircon-Anything-New", // caught by prefix sweep
+		"From",
+		"Sec-Ch-Ua",
+		"Sec-Ch-Ua-Platform",
 	}
 	for _, k := range removed {
 		h.Set(k, "leak")
@@ -56,3 +63,66 @@ func TestStripIdentifying(t *testing.T) {
 		}
 	}
 }
+
+func TestAnonymizeOverridesUserAgent(t *testing.T) {
+	for _, ua := range []string{"crawlkit-roles", "python-requests/2.32", "Go-http-client/1.1", ""} {
+		h := http.Header{}
+		if ua != "" {
+			h.Set("User-Agent", ua)
+		}
+		h.Set("X-Forwarded-For", "10.0.0.1")
+		anonymize(h, defaultUserAgent)
+		if got := h.Values("User-Agent"); len(got) != 1 || got[0] != defaultUserAgent {
+			t.Errorf("caller UA %q: outbound User-Agent = %q, want only the browser UA", ua, got)
+		}
+		if h.Get("X-Forwarded-For") != "" {
+			t.Errorf("anonymize must still strip forwarding headers")
+		}
+	}
+}
+
+func TestDefaultUserAgentIsBrowser(t *testing.T) {
+	if !strings.HasPrefix(defaultUserAgent, "Mozilla/5.0 (") || !strings.Contains(defaultUserAgent, "Firefox/") {
+		t.Errorf("default UA should be a plain Firefox string, got %q", defaultUserAgent)
+	}
+}
+
+// End to end: what the TARGET receives when a careless client goes through the
+// full handler. This is the no-leak contract, not just the helper.
+func TestHandlerTargetSeesOnlyBrowser(t *testing.T) {
+	var got http.Header
+	target := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		got = r.Header.Clone()
+	}))
+	defer target.Close()
+
+	cfg := Config{UserAgent: defaultUserAgent}
+	mgr := newProxyManager(1, 0, 0, time.Minute)
+	h := newHandler(cfg, []*http.Transport{{}}, mgr)
+
+	req := httptest.NewRequest(http.MethodGet, "/", nil)
+	req.Header.Set("X-Weircon-Random-Ip", "secret-key")
+	req.Header.Set("X-Weircon-Random-Ip-Redirect", target.URL+"/x")
+	req.Header.Set("User-Agent", "crawlkit-roles")
+	req.Header.Set("From", "someone@example.com")
+	req.Header.Set("X-Forwarded-For", "192.168.10.144")
+	req.Header.Set("X-Real-Ip", "192.168.10.144")
+	req.Header.Set("Sec-Ch-Ua", `"Chromium";v="125"`)
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status %d: %s", rec.Code, rec.Body.String())
+	}
+	if ua := got.Values("User-Agent"); len(ua) != 1 || ua[0] != defaultUserAgent {
+		t.Errorf("target saw User-Agent %q", ua)
+	}
+	for k, v := range got {
+		lk := strings.ToLower(k + ":" + strings.Join(v, ","))
+		for _, bad := range []string{"weircon", "crawlkit", "secret-key", "192.168.", "example.com", "forwarded", "sec-ch-ua", "from:"} {
+			if strings.Contains(lk, bad) {
+				t.Errorf("target saw identifying header %s: %v", k, v)
+			}
+		}
+	}
+}