perf: lazy-load @mdi/js to reduce Vite dev server memory usage

Replace `import * as mdi from '@mdi/js'` (loads ~5MB of SVG paths
synchronously into every HMR update) with a lazy async import that
loads once and caches. MdiIcon and IconPicker now use getMdiPath()
and getAllMdiNames() from the shared mdi-lookup module.

frontend/src/lib/components/MdiIcon.svelte

@@ -0,0 +1,9 @@
+<script lang="ts">
+	import { getMdiPath } from '$lib/mdi-lookup';
+
+	let { name = '', size = 18 } = $props<{ name: string; size?: number }>();
+</script>
+
+{#if name && getMdiPath(name)}
+	<svg viewBox="0 0 24 24" width={size} height={size} fill="currentColor"><path d={getMdiPath(name)} /></svg>
+{/if}