feat(ui): password strength meter with zxcvbn

Add PasswordStrengthMeter component showing visual bar and feedback.
Integrated in setup, settings change-password, and admin create-user forms.

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

src/lib/components/PasswordStrengthMeter.svelte

@@ -0,0 +1,36 @@
+<script lang="ts">
+	import zxcvbn from 'zxcvbn';
+
+	let { password = '' }: { password: string } = $props();
+
+	const result = $derived(password.length > 0 ? zxcvbn(password) : null);
+
+	const labels = ['Very weak', 'Weak', 'Fair', 'Strong', 'Very strong'];
+	const colors = [
+		'bg-red-500',
+		'bg-orange-500',
+		'bg-yellow-500',
+		'bg-lime-500',
+		'bg-green-500'
+	];
+</script>
+
+{#if result}
+	<div class="mt-1 mb-2" data-testid="password-strength">
+		<div class="flex gap-1">
+			{#each { length: 4 } as _, i}
+				<div
+					class="h-1 flex-1 rounded-full transition-colors {i <= result.score
+						? colors[result.score]
+						: 'bg-gray-200 dark:bg-gray-600'}"
+				></div>
+			{/each}
+		</div>
+		<p class="mt-1 text-xs text-gray-500 dark:text-gray-400">
+			{labels[result.score]}
+			{#if result.feedback.warning}
+				— {result.feedback.warning}
+			{/if}
+		</p>
+	</div>
+{/if}