feat(plugins): add plugin i18n foundation (Phase 0-1)

- Add i18n field to PluginManifest type in plugin-api package
- Create PluginI18nService for translation management
  - Load translations from file paths or cached content
  - Nested key lookup with dot notation (e.g., BUTTONS.SAVE)
  - Smart fallback: current language → English → key
  - Parameter interpolation with {{param}} syntax
  - Language switching via signals
  - Memory cleanup for unloaded plugins
- Add type validation for cached translation content
- Document language sync integration points

Part of plugin internationalization system implementation.
Tests will be added in Phase 8.

packages/plugin-api/src/types.ts

@@ -116,6 +116,9 @@ export interface PluginManifest {
   nodeScriptConfig?: PluginNodeScriptConfig;
   sidePanel?: boolean; // If true, plugin loads in right panel instead of route
   jsonSchemaCfg?: string; // Path to JSON schema file for plugin configuration relative to plugin root
+  i18n?: {
+    languages: string[]; // Array of supported language codes (e.g., ['en', 'de', 'fr'])
+  };
 }
 
 // Hook payload types