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/server/handlers_test.go

@@ -222,7 +222,27 @@ func TestDownloadEndpoints(t *testing.T) {
 	assert.Equal(t, http.StatusFound, rec.Code)
 	assert.Equal(t, "https://example.com/Player.exe", rec.Header().Get("Location"))
 
+	// A release without Linux files, like those before 1.9.
 	assert.Equal(t, http.StatusNotFound, e.do(http.MethodGet, "/api/v1/download/linux").Code)
+	assert.Equal(t, http.StatusNotFound, e.do(http.MethodGet, "/api/v1/download/linux-package").Code)
+}
+
+func TestDownloadLinuxBinaryAndPackage(t *testing.T) {
+	gitea := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Write([]byte(`{"id":1,"name":"v1.9.0","assets":[` +
+			`{"name":"MusicPlayer_1.9.0_linux.tar.gz","browser_download_url":"https://example.com/package"},` +
+			`{"name":"MusicPlayer_1.9.0.x86_64","browser_download_url":"https://example.com/binary"}]}`))
+	}))
+	t.Cleanup(gitea.Close)
+	e := newTestEnv(t, func(c *config.Config) { c.ReleasesURL = gitea.URL })
+
+	rec := e.do(http.MethodGet, "/api/v1/download/linux")
+	assert.Equal(t, http.StatusFound, rec.Code)
+	assert.Equal(t, "https://example.com/binary", rec.Header().Get("Location"), "the bare binary, for updating in place")
+
+	rec = e.do(http.MethodGet, "/api/v1/download/linux-package")
+	assert.Equal(t, http.StatusFound, rec.Code)
+	assert.Equal(t, "https://example.com/package", rec.Header().Get("Location"))
 }
 
 func TestDownloadUpstreamDown(t *testing.T) {