fix(xray): verify the release archive checksum before installing (#5396)

* fix(xray): verify the release archive checksum before installing

UpdateXray downloaded the Xray-core release zip and installed the binary
from it after only a TLS fetch, an HTTP-200 check and a size cap — the
archive itself was never verified, so a corrupted or tampered release
asset would be extracted and run as the panel's xray binary.

Verify the downloaded archive against the SHA2-256 published in the
release's .dgst sidecar (which XTLS ships next to every asset) before
installing, and abort the update on mismatch, a missing/short SHA2-256
entry, or an unreachable .dgst. The digest parser and fetch are covered by
tests, including the real .dgst line format ("SHA2-256= <hex>").

* address review: clearer warning + re-download guidance on checksum mismatch

Per review feedback on the PR: on a SHA-256 mismatch, surface a plain-language
warning that the downloaded archive is corrupted or differs from the official
release and that the user should exit and re-download, instead of a terse
"checksum mismatch" error. The install still aborts so a mismatched binary is
never run; the message now tells the user the safe next step.

internal/web/service/server_xray_checksum_test.go

@@ -0,0 +1,72 @@
+package service
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// A real XTLS .dgst sidecar (Xray-linux-64.zip.dgst, v26.3.27): lines are
+// "ALGO= <hex>", and the algorithm label is "SHA2-256", not "SHA256".
+const sampleXrayDgst = `# Hash Values
+
+MD5= ee4e2ff74948a9b464624b1cabc44409
+SHA1= b55b06e74e89083b9cedfdecf0d68b579cd2af72
+SHA2-256= 23cd9af937744d97776ee35ecad4972cf4b2109d1e0fe6be9930467608f7c8ae
+SHA2-512= e8bc40a0687cac184bbe4b5c1f047e69064ccedc489fb25e208889ae287bbf8736dff16b108d68fc00dc33edc8bb53502e47a9698a277f4f51b67b83d899e518
+`
+
+const wantSHA = "23cd9af937744d97776ee35ecad4972cf4b2109d1e0fe6be9930467608f7c8ae"
+
+func TestParseXrayDigestSHA256(t *testing.T) {
+	got, err := parseXrayDigestSHA256([]byte(sampleXrayDgst))
+	if err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+	if got != wantSHA {
+		t.Fatalf("sha = %q, want %q", got, wantSHA)
+	}
+}
+
+func TestParseXrayDigestSHA256_Errors(t *testing.T) {
+	for _, tc := range []struct {
+		name string
+		in   string
+	}{
+		{"no-sha256-line", "MD5= abc\nSHA1= def\n"},
+		{"malformed-short", "SHA2-256= deadbeef\n"},
+		{"empty", ""},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			if _, err := parseXrayDigestSHA256([]byte(tc.in)); err == nil {
+				t.Fatalf("%s: expected an error", tc.name)
+			}
+		})
+	}
+}
+
+func TestFetchXrayDigestSHA256(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, _ = w.Write([]byte(sampleXrayDgst))
+	}))
+	defer srv.Close()
+
+	got, err := (&ServerService{}).fetchXrayDigestSHA256(srv.Client(), srv.URL+"/Xray-linux-64.zip.dgst")
+	if err != nil {
+		t.Fatalf("fetch: %v", err)
+	}
+	if got != wantSHA {
+		t.Fatalf("sha = %q, want %q", got, wantSHA)
+	}
+}
+
+func TestFetchXrayDigestSHA256_HTTPError(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Error(w, "nope", http.StatusNotFound)
+	}))
+	defer srv.Close()
+
+	if _, err := (&ServerService{}).fetchXrayDigestSHA256(srv.Client(), srv.URL+"/missing.dgst"); err == nil {
+		t.Fatal("expected an error on HTTP 404")
+	}
+}