feat(skills): add skill sync method setting (symlink/copy)

- Add SyncMethod enum (Auto/Symlink/Copy) in Rust backend
- Implement sync_to_app_dir with symlink support (cross-platform)
- Add SkillSyncMethodSettings UI component (simplified 2-button selector)
- Add i18n support for zh/en/ja
- Replace copy_to_app with configurable sync_to_app_dir
- Add skill_sync_method field to AppSettings

User can now choose between symlink (disk space saving) or copy (best compatibility) in Settings > General.

src/components/settings/SettingsPage.tsx

@@ -35,6 +35,7 @@ import { LanguageSettings } from "@/components/settings/LanguageSettings";
 import { ThemeSettings } from "@/components/settings/ThemeSettings";
 import { WindowSettings } from "@/components/settings/WindowSettings";
 import { AppVisibilitySettings } from "@/components/settings/AppVisibilitySettings";
+import { SkillSyncMethodSettings } from "@/components/settings/SkillSyncMethodSettings";
 import { DirectorySettings } from "@/components/settings/DirectorySettings";
 import { ImportExportSection } from "@/components/settings/ImportExportSection";
 import { AboutSection } from "@/components/settings/AboutSection";
@@ -249,6 +250,12 @@ export function SettingsPage({
                     settings={settings}
                     onChange={handleAutoSave}
                   />
+                  <SkillSyncMethodSettings
+                    value={settings.skillSyncMethod ?? "auto"}
+                    onChange={(method) =>
+                      handleAutoSave({ skillSyncMethod: method })
+                    }
+                  />
                 </motion.div>
               ) : null}
             </TabsContent>