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/board/BoardCard.svelte

@@ -1,4 +1,5 @@
 <script lang="ts">
+	import { t } from 'svelte-i18n';
 	import DynamicIcon from '$lib/components/ui/DynamicIcon.svelte';
 
 	interface BoardSummary {
@@ -39,12 +40,12 @@
 				</h3>
 				{#if board.isDefault}
 					<span class="shrink-0 rounded bg-primary/15 px-1.5 py-0.5 text-xs text-primary">
-						Default
+						{$t('board.default')}
 					</span>
 				{/if}
 				{#if board.isGuestAccessible}
 					<span class="shrink-0 rounded bg-accent px-1.5 py-0.5 text-xs text-accent-foreground">
-						Guest
+						{$t('board.guest')}
 					</span>
 				{/if}
 			</div>
@@ -52,7 +53,7 @@
 				<p class="mt-1 line-clamp-2 text-sm text-muted-foreground">{board.description}</p>
 			{/if}
 			<p class="mt-2 text-xs text-muted-foreground/70">
-				{sectionCount} section{sectionCount === 1 ? '' : 's'}
+				{$t('board.sections_count', { values: { count: sectionCount } })}
 			</p>
 		</div>
 	</div>