Add GET /api/v1/download/linux-package

Redirects to the newest MusicPlayer release's _linux.tar.gz, which has the
icon, a menu entry and install.sh. /download/linux keeps serving the bare
.x86_64, which updating in place needs. Releases without the package get 404.

Closes #34

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/backend/releases_test.go

@@ -52,6 +52,26 @@ func TestReleaseService(t *testing.T) {
 	assert.ErrorIs(t, err, ErrNotFound)
 }
 
+func TestReleaseServiceLinuxBinaryAndPackage(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		json.NewEncoder(w).Encode(giteaRelease{Id: 1, Name: "v1.9.0", Assets: []giteaAsset{
+			{Name: "MusicPlayer_1.9.0.exe", DownloadUrl: "https://example.com/exe"},
+			{Name: "MusicPlayer_1.9.0_linux.tar.gz", DownloadUrl: "https://example.com/package"},
+			{Name: "MusicPlayer_1.9.0.x86_64", DownloadUrl: "https://example.com/binary"},
+		}})
+	}))
+	t.Cleanup(server.Close)
+	r := NewReleaseService(server.URL)
+	ctx := context.Background()
+
+	binary, err := r.AssetURL(ctx, ".x86_64")
+	require.NoError(t, err)
+	assert.Equal(t, "https://example.com/binary", binary)
+	pkg, err := r.AssetURL(ctx, "_linux.tar.gz")
+	require.NoError(t, err)
+	assert.Equal(t, "https://example.com/package", pkg)
+}
+
 func TestReleaseServiceCachesRelease(t *testing.T) {
 	server, calls := newGitea(t, http.StatusOK)
 	r := NewReleaseService(server.URL)