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/IconSprite.svelte

@@ -0,0 +1,13 @@
+<script lang="ts">
+	import { icons } from '$lib/icons/registry';
+</script>
+
+<!--
+	Единственный источник path-данных. Не использовать display:none — некоторые браузеры
+	тогда не рисуют <use>; прячем через нулевой размер и overflow.
+-->
+<svg width="0" height="0" class="absolute h-0 w-0 overflow-hidden" aria-hidden="true" focusable="false">
+	{#each Object.entries(icons) as [key, def] (key)}
+		<symbol id="i-{key}" viewBox="0 0 24 24">{@html def.path}</symbol>
+	{/each}
+</svg>