feat(frontend): replace emoji with inline Material Symbols icon system

Add a self-contained icon system: vendored Material Symbols registry, an
<Icon> component that references a single <symbol> sprite via <use>, plus
IconButton/IconSprite. Replace every emoji/Unicode glyph (logo, theme
switcher, cover placeholder, like/comment counts, lightbox controls, back
links) and add leading icons to key controls. Decorative icons are
aria-hidden; icon-only buttons get labels; theme toggles expose
aria-pressed; counters get accessible names.

Recorded in openspec change material-icons-ui (all tasks complete).

frontend/src/lib/components/Icon.svelte

@@ -0,0 +1,27 @@
+<script lang="ts">
+	import { icons, type IconName } from '$lib/icons/registry';
+
+	interface Props {
+		name: IconName;
+		label?: string;
+		class?: string;
+	}
+
+	let { name, label, class: className = 'size-5' }: Props = $props();
+
+	const known = $derived(name in icons);
+</script>
+
+{#if known}
+	<svg
+		viewBox="0 0 24 24"
+		fill="currentColor"
+		class={className}
+		role={label ? 'img' : undefined}
+		aria-label={label}
+		aria-hidden={label ? undefined : 'true'}
+		focusable="false"
+	>
+		<use href="#i-{name}" />
+	</svg>
+{/if}