feat(ui): settings page with user profile and admin user management

- Settings page: edit display name/email, change password
- Admin user management page: create/delete users, toggle roles
- Profile update API endpoint (PUT /api/auth/profile)
- Settings link added to sidebar navigation

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

src/lib/components/Layout/Sidebar.svelte

@@ -73,6 +73,23 @@
 			</li>
 		</ul>
 
+		<div class="mt-6 border-t border-gray-200 pt-4 dark:border-gray-700">
+			<ul class="space-y-1">
+				<li>
+					<a
+						href="/settings"
+						class="flex w-full items-center gap-3 rounded-full px-6 py-3 text-left text-sm text-gray-700 transition-colors hover:bg-gray-100 dark:text-gray-300 dark:hover:bg-gray-800"
+					>
+						<svg class="h-5 w-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+							<path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M10.325 4.317c.426-1.756 2.924-1.756 3.35 0a1.724 1.724 0 002.573 1.066c1.543-.94 3.31.826 2.37 2.37a1.724 1.724 0 001.066 2.573c1.756.426 1.756 2.924 0 3.35a1.724 1.724 0 00-1.066 2.573c.94 1.543-.826 3.31-2.37 2.37a1.724 1.724 0 00-2.573 1.066c-.426 1.756-2.924 1.756-3.35 0a1.724 1.724 0 00-2.573-1.066c-1.543.94-3.31-.826-2.37-2.37a1.724 1.724 0 00-1.066-2.573c-1.756-.426-1.756-2.924 0-3.35a1.724 1.724 0 001.066-2.573c-.94-1.543.826-3.31 2.37-2.37.996.608 2.296.07 2.572-1.065z" />
+							<path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M15 12a3 3 0 11-6 0 3 3 0 016 0z" />
+						</svg>
+						Settings
+					</a>
+				</li>
+			</ul>
+		</div>
+
 		{#if $allTags.length > 0}
 			<div class="mt-6 border-t border-[var(--border-subtle)] pt-4">
 				<h3 class="px-6 text-xs font-semibold uppercase tracking-wider text-[var(--text-muted)]">Tags</h3>