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/routes/invites/[token]/+page.svelte

@@ -2,6 +2,7 @@
 	import { page } from '$app/stores';
 	import { goto } from '$app/navigation';
 	import { api } from '$lib/api';
+	import Icon from '$lib/components/Icon.svelte';
 	import { auth } from '$lib/stores/auth.svelte';
 
 	const token = $derived($page.params.token);
@@ -43,13 +44,20 @@
 				type="button"
 				disabled={busy}
 				onclick={accept}
-				class="bg-primary text-primary-fg rounded-lg px-5 py-2.5 font-medium"
+				class="bg-primary text-primary-fg inline-flex items-center gap-2 rounded-lg px-5 py-2.5 font-medium"
 			>
+				<Icon name="check" class="size-5" />
 				{busy ? 'Принятие…' : 'Принять приглашение'}
 			</button>
 		{:else}
 			<p class="mb-4 text-sm">Войдите или зарегистрируйтесь, чтобы принять приглашение.</p>
-			<a href="/login" class="bg-primary text-primary-fg rounded-lg px-5 py-2.5 font-medium">Войти</a>
+			<a
+				href="/login"
+				class="bg-primary text-primary-fg inline-flex items-center gap-2 rounded-lg px-5 py-2.5 font-medium"
+			>
+				<Icon name="login" class="size-5" />
+				Войти
+			</a>
 		{/if}
 	{:else}
 		<p class="text-text-muted">Загрузка…</p>