feat(frontend): Velxio iframe fullscreen + playground route + SvelteKit service worker

- VelxioTab: add allowfullscreen to Velxio editor iframe
- lesson.css: flex/shrink fixes for .velxio-iframe on mobile
- +layout.svelte: hide Navbar/Footer on /playground full-viewport route
- Add /playground route: Velxio editor iframe + DeployFAB bridge
- Migrate static sw.js to SvelteKit module service-worker.ts

frontend/src/routes/+layout.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 	import { onMount } from 'svelte';
+	import { page } from '$app/stores';
 	import { auth } from '$stores/auth';
 	import { theme } from '$stores/theme';
 	import Navbar from '$components/Navbar.svelte';
@@ -12,12 +13,19 @@
 		theme.init();
 		auth.init();
 	});
+
+	// Suppress Navbar/Footer on full-viewport routes (e.g. /playground)
+	let isFullViewport = $derived($page.url.pathname === '/playground');
 </script>
 
-<Navbar />
+{#if !isFullViewport}
+	<Navbar />
+{/if}
 
-<main class="container" style="flex: 1; padding-block: 1.5rem;">
+<main class="container" style="flex: 1; padding-block: {isFullViewport ? '0' : '1.5rem'}; overflow: {isFullViewport ? 'hidden' : 'visible'};">
 	{@render children()}
 </main>
 
-<Footer />
+{#if !isFullViewport}
+	<Footer />
+{/if}

frontend/src/routes/lesson/[slug]/VelxioTab.svelte

@@ -33,6 +33,7 @@
 		class="velxio-iframe"
 		src="/velxio/editor?embed=true{hasArduinoCode ? '' : '&hideEditor=true'}&lockComponents=true"
 		onload={(e) => onSetupBridge(e.currentTarget as HTMLIFrameElement)}
-		allow="cross-origin-isolated"
+		allow="cross-origin-isolated; fullscreen"
+		allowfullscreen
 	></iframe>
 {/if}

frontend/src/routes/lesson/[slug]/lesson.css

@@ -204,7 +204,8 @@
 	box-shadow: 0 -4px 20px rgba(0, 0, 0, 0.15);
 	display: flex;
 	flex-direction: column;
-	overflow: hidden;
+	overflow-y: auto;
+	overflow-x: hidden;
 	transition: height 0.3s cubic-bezier(0.4, 0, 0.2, 1), border-radius 0.2s ease;
 }
 .editor-area.mobile-hidden {
@@ -244,6 +245,10 @@
 .editor-area.mobile-full .cm-scroller {
 	flex: 1;
 }
+.editor-area.mobile-full .velxio-iframe {
+	flex: 1;
+	min-height: 0;
+}
 
 /* ── Tab panels ────────────────────────────────────────── */
 .tab-panel {
@@ -300,9 +305,25 @@
 .velxio-iframe {
 	flex: 1;
 	width: 100%;
-	min-height: 400px;
+	min-height: 0;              /* CHANGED: was 400px — allow shrink in flex */
 	border: none;
 	border-radius: var(--radius);
+	touch-action: pan-y;               /* allow vertical pan inside iframe */
+	overscroll-behavior: contain;      /* prevent overscroll bounce */
+}
+@media (max-width: 768px) {
+	.velxio-iframe {
+		min-height: 0;    /* CHANGED: was 200px — let flex sizing work fully */
+	}
+	/* Ensure sheet never exceeds visual viewport when keyboard open */
+	.editor-area.mobile-sheet {
+		max-height: 100vh;  /* fallback for browsers without dvh */
+		max-height: 100dvh; /* dynamic viewport height */
+	}
+	.editor-area.mobile-full {
+		height: 100dvh;
+		max-height: 100dvh;
+	}
 }
 .velxio-fallback {
 	padding: 2rem;

frontend/src/routes/playground/+page.svelte

@@ -0,0 +1,97 @@
+<script lang="ts">
+	import { onMount, onDestroy } from 'svelte';
+	import DeployFAB from '$components/DeployFAB.svelte';
+
+	let velxioIframe = $state<HTMLIFrameElement | null>(null);
+	type DeployFABHandle = { setHex: (hex: string | null) => void };
+	let fabComponent = $state<DeployFABHandle | null>(null);
+
+	const iframeSrc = '/velxio/editor?embed=true&desktopLayout=true';
+
+	function handleMessage(e: MessageEvent) {
+		if (!e.data?.type) return;
+		if (e.data.type === 'velxio:hex_ready') {
+			fabComponent?.setHex(e.data.hex);
+		}
+	}
+
+	function pushSerialToIframe(text: string) {
+		if (!velxioIframe?.contentWindow) return;
+		velxioIframe.contentWindow.postMessage({
+			type: 'elemes:push_serial',
+			text,
+			boardId: null
+		}, window.location.origin);
+	}
+
+	function notifySerialStart() {
+		velxioIframe?.contentWindow?.postMessage({ type: 'elemes:start_hardware_serial' }, window.location.origin);
+	}
+
+	function notifySerialStop() {
+		velxioIframe?.contentWindow?.postMessage({ type: 'elemes:stop_hardware_serial' }, window.location.origin);
+	}
+
+	onMount(() => {
+		window.addEventListener('message', handleMessage);
+	});
+
+	onDestroy(() => {
+		window.removeEventListener('message', handleMessage);
+	});
+</script>
+
+<svelte:head>
+	<title>Developer Playground — Velxio</title>
+</svelte:head>
+
+<div class="playground-page">
+	<iframe
+		bind:this={velxioIframe}
+		src={iframeSrc}
+		title="Velxio Editor"
+		allow="bluetooth; clipboard-read; clipboard-write; fullscreen"
+		class="velxio-iframe"
+		allowfullscreen
+	></iframe>
+
+	<DeployFAB
+		bind:this={fabComponent}
+		{velxioIframe}
+		onPushSerial={pushSerialToIframe}
+		onSerialStart={notifySerialStart}
+		onStopSerial={notifySerialStop}
+	/>
+</div>
+
+<style>
+	.playground-page {
+		width: 100%;
+		height: 100dvh;
+		overflow-y: auto;
+		overflow-x: hidden;
+		display: flex;
+		flex-direction: column;
+	}
+
+	.velxio-iframe {
+		width: 100%;
+		flex: 1;
+		min-height: 0;           /* critical: allows flex child to shrink below content size */
+		border: none;
+		display: block;
+		touch-action: pan-y;     /* allow vertical pan inside iframe */
+	}
+
+	@media (max-width: 768px) {
+		.playground-page {
+			overflow-y: auto;
+			height: auto;        /* allow page to grow */
+			min-height: 100dvh;  /* but always at least fill viewport */
+		}
+		.velxio-iframe {
+			max-height: none;    /* iframe can grow taller than viewport */
+			height: auto;
+		}
+	}
+</style>

frontend/src/routes/playground/+page.ts

@@ -0,0 +1,3 @@
+// Disable SSR — Web Bluetooth & iframe PostMessage need client-side
+export const prerender = false;
+export const ssr = false;

frontend/src/service-worker.ts

@@ -1,5 +1,7 @@
-// static/sw.js
-const CACHE_VERSION = 'elemes-v14';
+// src/service-worker.ts — SvelteKit Service Worker
+import { APP_VERSION } from '$lib/version';
+
+const CACHE_VERSION = `elemes-v${APP_VERSION}`;
 const STATIC_CACHE = `${CACHE_VERSION}-static`;
 const API_CACHE = `${CACHE_VERSION}-api`;
 const ASSET_CACHE = `${CACHE_VERSION}-assets`;
@@ -75,9 +77,9 @@ self.addEventListener('fetch', (event) => {
     return;
   }
 
-  // StaleWhileRevalidate: HTML pages
+  // NetworkFirst: HTML pages (new routes need network first)
   if (request.headers.get('accept')?.includes('text/html')) {
-    event.respondWith(staleWhileRevalidate(request, STATIC_CACHE));
+    event.respondWith(networkFirst(request, STATIC_CACHE, 1440));
     return;
   }
 
@@ -90,7 +92,7 @@ self.addEventListener('fetch', (event) => {
 
 // ── Helpers ──────────────────────────────────────────────────────
 
-async function cacheFirst(request, cacheName, maxAgeDays = 7) {
+async function cacheFirst(request: Request, cacheName: string, maxAgeDays = 7): Promise<Response> {
   const cache = await caches.open(cacheName);
   const cached = await cache.match(request);
   if (cached) return cached;
@@ -99,7 +101,7 @@ async function cacheFirst(request, cacheName, maxAgeDays = 7) {
   return response;
 }
 
-async function networkFirst(request, cacheName, maxAgeMinutes = 60) {
+async function networkFirst(request: Request, cacheName: string, maxAgeMinutes = 60): Promise<Response> {
   const cache = await caches.open(cacheName);
   try {
     const response = await fetch(request);
@@ -115,7 +117,7 @@ async function networkFirst(request, cacheName, maxAgeMinutes = 60) {
   }
 }
 
-async function staleWhileRevalidate(request, cacheName) {
+async function staleWhileRevalidate(request: Request, cacheName: string): Promise<Response> {
   const cache = await caches.open(cacheName);
   const cached = await cache.match(request);
   const networkFetch = fetch(request).then(response => {