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/PhotoGrid.svelte

@@ -1,4 +1,5 @@
 <script lang="ts">
+	import Icon from './Icon.svelte';
 	import type { Photo } from '$lib/types';
 
 	interface Props {
@@ -34,9 +35,22 @@
 						</span>
 					{/if}
 					<span
-						class="bg-bg/70 text-text absolute right-1 bottom-1 rounded px-1.5 py-0.5 text-xs"
+						class="bg-bg/70 text-text absolute right-1 bottom-1 flex items-center gap-1.5 rounded px-1.5 py-0.5 text-xs"
 					>
-						♥ {photo.likes_count} · 💬 {photo.comments_count}
+						<span class="inline-flex items-center gap-0.5" aria-label="{photo.likes_count} лайков">
+							<Icon
+								name={photo.liked_by_me ? 'favorite' : 'favorite_outline'}
+								class="size-3.5"
+							/>
+							{photo.likes_count}
+						</span>
+						<span
+							class="inline-flex items-center gap-0.5"
+							aria-label="{photo.comments_count} комментариев"
+						>
+							<Icon name="mode_comment" class="size-3.5" />
+							{photo.comments_count}
+						</span>
 					</span>
 				</button>
 			</li>