feat(nav): add emoji icons to all nav components

Renders emoji icons in MobileTabBar (stacked above label),
TabletNavBar (inline), and DesktopSidebar (16px, 20px column).

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

frontend/src/lib/nav/DesktopSidebar.svelte

@@ -32,6 +32,7 @@
 						? 'bg-[var(--green-tint)] text-[var(--green-dark)] font-medium'
 						: ''}"
 				>
+					<span class="w-[20px] text-[16px] text-center">{item.icon}</span>
 					{item.label}
 				</a>
 			{/each}