feat(frontend): add stealth overlay

frontend/src/lib/components/StealthOverlay.svelte

@@ -0,0 +1,52 @@
+<script lang="ts">
+	let stealthMode = $state(false);
+	let lastTap = $state({ time: 0, x: 0, y: 0 });
+
+	function isInCenterZone(x: number, y: number): boolean {
+		const w = window.innerWidth;
+		const h = window.innerHeight;
+		return x > w * 0.3 && x < w * 0.7 && y > h * 0.3 && y < h * 0.7;
+	}
+
+	function handleTouchEnd(e: TouchEvent) {
+		if (e.touches.length > 0) return;
+
+		const touch = e.changedTouches[0];
+		const now = Date.now();
+		const x = touch.clientX;
+		const y = touch.clientY;
+
+		if (!isInCenterZone(x, y)) {
+			lastTap = { time: 0, x: 0, y: 0 };
+			return;
+		}
+
+		const timeDiff = now - lastTap.time;
+		const distX = Math.abs(x - lastTap.x);
+		const distY = Math.abs(y - lastTap.y);
+
+		if (timeDiff < 300 && distX < 50 && distY < 50) {
+			stealthMode = !stealthMode;
+			lastTap = { time: 0, x: 0, y: 0 };
+			e.preventDefault();
+		} else {
+			lastTap = { time: now, x, y };
+		}
+	}
+</script>
+
+<svelte:document ontouchend={handleTouchEnd} />
+
+{#if stealthMode}
+	<div class="stealth-overlay" ontouchend={handleTouchEnd}></div>
+{/if}
+
+<style>
+	.stealth-overlay {
+		position: fixed;
+		inset: 0;
+		z-index: 9999;
+		background: #000;
+		touch-action: manipulation;
+	}
+</style>

frontend/src/routes/[mnemonic]/+page.svelte

@@ -7,6 +7,7 @@
 	import ChatMessage from '$lib/components/ChatMessage.svelte';
 	import ChatInput from '$lib/components/ChatInput.svelte';
 	import FollowUpButtons from '$lib/components/FollowUpButtons.svelte';
+	import StealthOverlay from '$lib/components/StealthOverlay.svelte';
 
 	const usePolling = getContext<boolean>('convex-use-polling') ?? false;
 	let mnemonic = $derived(page.params.mnemonic);
@@ -185,4 +186,6 @@
 			↓
 		</button>
 	{/if}
+
+	<StealthOverlay />
 </div>