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/widget/EmbedWidget.svelte

@@ -0,0 +1,50 @@
+<script lang="ts">
+	interface EmbedConfig {
+		url: string;
+		height: number;
+		sandbox?: string;
+	}
+
+	interface Props {
+		config: EmbedConfig;
+	}
+
+	let { config }: Props = $props();
+
+	let loading = $state(true);
+
+	const iframeHeight = $derived(config.height || 300);
+	const sandboxValue = $derived(config.sandbox || 'allow-scripts allow-same-origin');
+
+	function handleLoad() {
+		loading = false;
+	}
+</script>
+
+<div class="flex flex-col rounded-xl border border-border bg-card">
+	<div class="relative" style="height: {iframeHeight}px;">
+		{#if loading}
+			<div class="absolute inset-0 flex items-center justify-center bg-muted/50">
+				<div class="flex items-center gap-2 text-sm text-muted-foreground">
+					<svg
+						class="h-4 w-4 animate-spin"
+						xmlns="http://www.w3.org/2000/svg"
+						fill="none"
+						viewBox="0 0 24 24"
+					>
+						<circle class="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" stroke-width="4" />
+						<path class="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4z" />
+					</svg>
+					Loading...
+				</div>
+			</div>
+		{/if}
+		<iframe
+			src={config.url}
+			title="Embedded content"
+			sandbox={sandboxValue}
+			class="h-full w-full rounded-xl border-0"
+			onload={handleLoad}
+		></iframe>
+	</div>
+</div>