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/+layout.svelte

@@ -3,6 +3,8 @@
 	import { goto } from '$app/navigation';
 	import '../app.css';
 	import ThemeSwitcher from '$lib/components/ThemeSwitcher.svelte';
+	import Icon from '$lib/components/Icon.svelte';
+	import IconSprite from '$lib/components/IconSprite.svelte';
 	import { auth } from '$lib/stores/auth.svelte';
 	import { theme } from '$lib/stores/theme.svelte';
 	import { authApi } from '$lib/api';
@@ -28,20 +30,33 @@
 	}
 </script>
 
+<IconSprite />
 <div class="bg-bg text-text min-h-screen">
 	<header class="border-border bg-bg-elevated border-b">
 		<div class="mx-auto flex max-w-6xl items-center justify-between gap-4 px-4 py-3">
-			<a href="/" class="text-lg font-semibold">📷 Galo</a>
+			<a href="/" class="inline-flex items-center gap-2 text-lg font-semibold">
+				<Icon name="photo_camera" />
+				Galo
+			</a>
 			<div class="flex items-center gap-3">
 				{#if auth.isAuthenticated}
 					<span class="text-text-muted hidden text-sm sm:inline">{auth.user?.name ?? ''}</span>
 					<button
 						type="button"
-						class="text-text-muted hover:text-text text-sm"
-						onclick={logout}>Выйти</button
+						class="text-text-muted hover:text-text inline-flex items-center gap-1.5 text-sm"
+						onclick={logout}
 					>
+						<Icon name="logout" class="size-4" />
+						Выйти
+					</button>
 				{:else}
-					<a href="/login" class="text-text-muted hover:text-text text-sm">Войти</a>
+					<a
+						href="/login"
+						class="text-text-muted hover:text-text inline-flex items-center gap-1.5 text-sm"
+					>
+						<Icon name="login" class="size-4" />
+						Войти
+					</a>
 				{/if}
 				<ThemeSwitcher />
 			</div>