<misc> UpgradeWindow: 调整 UpdateAvailable 布局

- 按钮移到右下角：Later 在左侧，Download 在最右侧
- 按钮间距 8px
- 版本信息格式改为 current: vX / → latest: vY

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

Author: Ethan

mgit-gui/src/ui/windows/upgrade.rs

@@ -113,15 +113,17 @@ impl super::View for UpgradeWindow {
                 } => {
                     ui.label("");
                     ui.heading("New version available!");
-                    ui.label(format!("v{latest}  →  current: v{current}"));
+                    ui.label(format!("current: v{current}"));
+                    ui.label(format!("    → latest: v{latest}"));
                     ui.label("");
-                    ui.horizontal(|ui| {
-                        if ui.button("  Download  ").clicked() {
-                            self.action = Some(UpgradeAction::Download);
-                        }
-                        if ui.button("  Later  ").clicked() {
+                    ui.with_layout(egui::Layout::right_to_left(egui::Align::BOTTOM), |ui| {
+                        ui.spacing_mut().item_spacing = egui::vec2(8.0, 0.0);
+                        if ui.button("Later").clicked() {
                             self.action = Some(UpgradeAction::Close);
                         }
+                        if ui.button("Download").clicked() {
+                            self.action = Some(UpgradeAction::Download);
+                        }
                     });
                 }