T1.7: Create src/platform/desktop; move desktop integration

Move desktop_linux.go and desktop_other.go from src/core to the new
src/platform/desktop package. Update src/gui to import and use the
new package location.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/platform/desktop/desktop_linux.go

@@ -0,0 +1,65 @@
+//go:build linux
+
+package desktop
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strconv"
+)
+
+func InstallDesktopIntegration(appID string, executablePath string, icon []byte) (string, error) {
+	dataHome, err := xdgDataHome()
+	if err != nil {
+		return "", err
+	}
+
+	// The taskbar can only show the application icon reliably when the desktop
+	// environment can match the window app id to an installed .desktop file and
+	// icon. Use the user's XDG data directory so portable builds do not need root
+	// access or a package manager install step.
+	iconPath := filepath.Join(dataHome, "icons", "hicolor", "256x256", "apps", "gosentry.png")
+	if err := writeUserFile(iconPath, icon, 0o644); err != nil {
+		return "", err
+	}
+
+	desktopPath := filepath.Join(dataHome, "applications", appID+".desktop")
+	desktopFile := fmt.Sprintf(`[Desktop Entry]
+Type=Application
+Name=GoSentry
+Comment=GoSentry desktop scheduler
+Exec=%s
+Icon=%s
+Terminal=false
+Categories=Utility;
+StartupWMClass=%s
+`, quoteDesktopExec(executablePath), iconPath, appID)
+	if err := writeUserFile(desktopPath, []byte(desktopFile), 0o644); err != nil {
+		return "", err
+	}
+	return iconPath, nil
+}
+
+func quoteDesktopExec(path string) string {
+	return strconv.Quote(path)
+}
+
+func xdgDataHome() (string, error) {
+	dataHome := os.Getenv("XDG_DATA_HOME")
+	if dataHome == "" {
+		home, err := os.UserHomeDir()
+		if err != nil {
+			return "", err
+		}
+		dataHome = filepath.Join(home, ".local", "share")
+	}
+	return dataHome, nil
+}
+
+func writeUserFile(path string, data []byte, perm os.FileMode) error {
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		return err
+	}
+	return os.WriteFile(path, data, perm)
+}

src/platform/desktop/desktop_other.go

@@ -0,0 +1,7 @@
+//go:build !linux
+
+package desktop
+
+func InstallDesktopIntegration(appID string, executablePath string, icon []byte) (string, error) {
+	return "", nil
+}