UAT-3: browser gate (hard stop) + iPad Safari fix

Spec CAPTURE.md §Browsers: Phone iOS Safari / Android Chrome, PC
Chrome / Edge / Firefox, others hard stop.

- browserGate.js: UA detection + allowlist (pure, DOM-free)
- +layout.svelte: renders hard-stop panel when unsupported
- iPadOS 13+ reports as Macintosh — added Safari rule (Version/ +
  Safari/ without Chrome/) so iPad Safari is not blocked
- 27 unit tests incl. iPad Safari regression
- 107/107 full suite green

frontend/src/routes/+layout.svelte

@@ -1,13 +1,71 @@
 <script>
 	import '../app.css';
+	import { checkBrowserGate } from '$lib/core/browserGate.js';
+
+	// UAT-3: hard-stop browser gate (CAPTURE.md §Browsers).
+	// Client-side only: the gate must be invisible to crawlers/bots
+	// and to any non-JS context, and the app must not render in an
+	// unsupported browser.
+	const gate = typeof navigator !== 'undefined' ? checkBrowserGate(navigator.userAgent) : null;
+	let unsupported = $state(gate ? !gate.supported : false);
+	let browserName = $state(gate ? gate.browser.name : 'this browser');
 </script>
 
 <svelte:head>
 	<title>SlabUploader — Milestone 1</title>
 </svelte:head>
 
-<main>
-	<h1>SlabUploader</h1>
-	<p>Milestone 1 — Capture (browser-only)</p>
-	<slot />
-</main>
+{#if unsupported}
+	<!-- Hard stop: the app does NOT render in an unsupported browser. -->
+	<div class="gate">
+		<h1>Unsupported browser</h1>
+		<p>SlabUploader does not work in <strong>{browserName}</strong>.</p>
+		<p class="sub">Use one of the supported clients:</p>
+		<ul>
+			<li>Phone — iOS Safari, Android Chrome</li>
+			<li>PC — Chrome, Edge, Firefox</li>
+		</ul>
+	</div>
+{:else}
+	<main>
+		<h1>SlabUploader</h1>
+		<p>Milestone 1 — Capture (browser-only)</p>
+		<slot />
+	</main>
+{/if}
+
+<style>
+	.gate {
+		min-height: 100vh;
+		display: flex;
+		align-items: center;
+		justify-content: center;
+		max-width: 32rem;
+		margin: 1rem auto;
+		padding: 2rem;
+		border: 2px solid #b00020;
+		border-radius: 0.5rem;
+		text-align: center;
+	}
+
+	.gate h1 {
+		color: #b00020;
+		margin: 0 0 1rem;
+	}
+
+	.gate .sub {
+		margin: 1rem 0 0.5rem;
+	}
+
+	.gate ul {
+		text-align: left;
+		margin: 0;
+		padding: 0;
+		list-style: none;
+	}
+
+	.gate li {
+		margin: 0.25rem 0;
+	}
+</style>
+