refactor(observability): lower trace sample rate, add DSN comment, improve status visibility

- Lower tracesSampleRate from 1.0 to 0.1 in both hooks (errors still captured
  at 100%; trace volume reduced for self-hosted GlitchTip on shared VPS)
- Add comment explaining VITE_SENTRY_DSN is a write-only ingest key, safe in
  client bundle — prevents accidental rotation as if it were a password
- Restore HTTP status code prominence: text-4xl font-bold (was text-xs text-ink-3)
- Add min-w-[44px] to copy button for WCAG 2.2 minimum touch target

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/+error.svelte

@@ -29,7 +29,7 @@ function copyId() {
 	<p class="mb-8 font-sans text-sm text-ink-2">
 		{page.error?.message ?? m.error_internal_error()}
 	</p>
-	<p class="font-sans text-xs tracking-widest text-ink-3 uppercase">{page.status}</p>
+	<p class="mb-4 font-mono text-4xl font-bold text-ink">{page.status}</p>
 
 	{#if page.error?.errorId}
 		<div class="mt-6 flex flex-col items-center gap-3">
@@ -42,7 +42,7 @@ function copyId() {
 				{page.error.errorId}
 			</code>
 			<button
-				class="min-h-[44px] rounded-sm bg-brand-navy px-5 py-2 font-sans text-sm text-white transition-colors hover:opacity-90 focus-visible:ring-2 focus-visible:ring-brand-navy focus-visible:ring-offset-2"
+				class="min-h-[44px] min-w-[44px] rounded-sm bg-brand-navy px-5 py-2 font-sans text-sm text-white transition-colors hover:opacity-90 focus-visible:ring-2 focus-visible:ring-brand-navy focus-visible:ring-offset-2"
 				onclick={copyId}
 				aria-label={m.error_copy_id_label()}
 			>