Implement Mihomo external-controller support in configuration and gateway

- Added optional Mihomo configuration fields in `config.compose.yaml` and `config.example.yaml` for enhanced integration with the Mihomo external-controller.
- Updated the `Gateway` to handle Mihomo API requests, including proxying and error handling for Mihomo-specific endpoints.
- Enhanced the documentation in `GATEWAY_RUN.md` to guide users on configuring Mihomo integration.
- Introduced new utility functions in the web client for interacting with Mihomo API endpoints, improving the overall user experience.
- Updated the sidebar in the Svelte components to include a link to the Mihomo section, enhancing navigation.

internal/proxy/mihomo_test.go

@@ -0,0 +1,37 @@
+package proxy
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"testing"
+)
+
+func TestMihomoForwardRewritesPathAndAuth(t *testing.T) {
+	target, err := url.Parse("http://127.0.0.1:9090")
+	if err != nil {
+		t.Fatal(err)
+	}
+	cap := &captureTransport{}
+	h := NewMihomoForward(target, "/api/mtg/mihomo", "Bearer testsecret", cap, nil)
+
+	req, err := http.NewRequestWithContext(context.Background(), http.MethodGet, "http://gw/api/mtg/mihomo/proxies", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	req.Header.Set("Authorization", "Bearer client-should-not-forward")
+	h.ServeHTTP(httptest.NewRecorder(), req)
+
+	if cap.got == nil {
+		t.Fatal("no outgoing request captured")
+	}
+	if got := cap.got.Header.Get("Authorization"); got != "Bearer testsecret" {
+		t.Fatalf("Authorization: got %q want Bearer testsecret", got)
+	}
+	want, err := url.Parse("http://127.0.0.1:9090/proxies")
+	if err != nil {
+		t.Fatal(err)
+	}
+	assertSameURL(t, cap.got.URL, want)
+}