T1.6: Create src/platform/autostart; move autostart logic

Move autostart_*.go and tests from src/core to src/platform/autostart.
Update src/gui/app.go to call autostart.SetAutostart / autostart.AutostartStatus.
Add quoteDesktopExec to desktop_linux.go (was co-located in autostart_linux.go).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/platform/autostart/autostart_linux_test.go

@@ -0,0 +1,57 @@
+//go:build linux
+
+package autostart
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"gitea.mixdep.ru/mix/gosentry/src/domain"
+)
+
+func TestLinuxAutostartStartsInTray(t *testing.T) {
+	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+
+	executablePath := "/opt/Go Sentry/gosentry"
+	if err := SetAutostart(true, executablePath, "/opt/Go Sentry/gosentry.png"); err != nil {
+		t.Fatalf("enable autostart: %v", err)
+	}
+
+	desktopPath, err := autostartDesktopPath()
+	if err != nil {
+		t.Fatalf("resolve desktop path: %v", err)
+	}
+	data, err := os.ReadFile(desktopPath)
+	if err != nil {
+		t.Fatalf("read desktop entry: %v", err)
+	}
+
+	expectedExec := "Exec=" + quoteDesktopExec(executablePath) + " " + domain.StartInTrayArgument
+	if !strings.Contains(string(data), expectedExec) {
+		t.Fatalf("desktop entry does not start in tray: %s", data)
+	}
+}
+
+func TestLinuxAutostartRemovesLegacyDesktopEntry(t *testing.T) {
+	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+
+	legacyPath, err := legacyAutostartDesktopPath()
+	if err != nil {
+		t.Fatalf("resolve legacy desktop path: %v", err)
+	}
+	if err := os.MkdirAll(filepath.Dir(legacyPath), 0o755); err != nil {
+		t.Fatalf("create legacy desktop directory: %v", err)
+	}
+	if err := os.WriteFile(legacyPath, []byte("[Desktop Entry]\nName=PySentry\n"), 0o644); err != nil {
+		t.Fatalf("write legacy desktop entry: %v", err)
+	}
+
+	if err := SetAutostart(true, "/opt/gosentry/gosentry", ""); err != nil {
+		t.Fatalf("enable autostart: %v", err)
+	}
+	if _, err := os.Stat(legacyPath); !os.IsNotExist(err) {
+		t.Fatalf("legacy desktop entry still exists or cannot be checked: %v", err)
+	}
+}