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/lib/schemas/settings.ts

@@ -25,6 +25,9 @@ export const settingsSchema = z.object({
   currentProviderClaude: z.string().optional(),
   currentProviderCodex: z.string().optional(),
   currentProviderGemini: z.string().optional(),
+
+  // Skill 同步设置
+  skillSyncMethod: z.enum(["auto", "symlink", "copy"]).optional(),
 });
 
 export type SettingsFormData = z.infer<typeof settingsSchema>;