feat(frontend): fullscreen toggle and album back button

- add fullscreen/fullscreen_exit icons and a top-bar toggle (right of themes)
- add back arrow left of the album title to return to the albums overview

frontend/src/lib/components/AppShell.svelte

@@ -24,13 +24,29 @@
 	let railWidth = $state(RAIL.def);
 	let panelWidth = $state(PANEL.def);
 
+	let isFullscreen = $state(false);
+
 	onMount(() => {
 		const r = parseInt(localStorage.getItem(RAIL.key) ?? '', 10);
 		if (r >= RAIL.min && r <= RAIL.max) railWidth = r;
 		const p = parseInt(localStorage.getItem(PANEL.key) ?? '', 10);
 		if (p >= PANEL.min && p <= PANEL.max) panelWidth = p;
+
+		const onFs = () => (isFullscreen = !!document.fullscreenElement);
+		document.addEventListener('fullscreenchange', onFs);
+		onFs();
+		return () => document.removeEventListener('fullscreenchange', onFs);
 	});
 
+	async function toggleFullscreen() {
+		try {
+			if (document.fullscreenElement) await document.exitFullscreen();
+			else await document.documentElement.requestFullscreen();
+		} catch {
+			/* недоступно */
+		}
+	}
+
 	function startResize(side: 'left' | 'right', ev: PointerEvent) {
 		ev.preventDefault();
 		const cfg = side === 'left' ? RAIL : PANEL;
@@ -134,6 +150,16 @@
 				</a>
 			{/if}
 			<ThemeSwitcher />
+			<button
+				type="button"
+				class="text-text-muted hover:bg-bg-muted hover:text-text inline-flex items-center justify-center rounded-md p-1.5"
+				aria-label={isFullscreen ? 'Выйти из полноэкранного режима' : 'На весь экран'}
+				title={isFullscreen ? 'Выйти из полноэкранного режима' : 'На весь экран'}
+				aria-pressed={isFullscreen}
+				onclick={toggleFullscreen}
+			>
+				<Icon name={isFullscreen ? 'fullscreen_exit' : 'fullscreen'} />
+			</button>
 		</div>
 	</header>