feat(phase2): localization EN/RU + additional widget types

- Add svelte-i18n with 224 translation keys (English + Russian)
- Language switcher in header (EN/RU toggle, persists to localStorage)
- Extract all hardcoded strings from 37 component/page files
- Add 4 new widget types: Bookmark, Note (markdown), Embed (iframe), Status
- WidgetRenderer dispatches by type, WidgetGrid supports full-width widgets
- Type-specific config forms in board editor
- Install marked for markdown rendering

src/lib/components/layout/MainLayout.svelte

@@ -1,4 +1,5 @@
 <script lang="ts">
+	import { t } from 'svelte-i18n';
 	import type { Snippet } from 'svelte';
 	import Sidebar from './Sidebar.svelte';
 	import Header from './Header.svelte';
@@ -40,7 +41,7 @@
 			type="button"
 			class="fixed inset-0 z-30 bg-black/50"
 			onclick={() => ui.closeMobileSidebar()}
-			aria-label="Close sidebar"
+			aria-label={$t('sidebar.close')}
 		></button>
 	{/if}