feat(web): add theme system with Obsidian, Paper, and Midnight themes

Three holistic design directions with CSS custom properties, a theme
store persisted to localStorage, and a live switcher in both the header
(cycle button) and settings page (card selector). Also fixes checkbox
checkmark alignment and adds back navigation from settings.

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

opal-web/src/lib/components/InputBar.svelte

@@ -2,7 +2,7 @@
 	import PropertyPills from './PropertyPills.svelte';
 
 	/**
-	 * @type {(input: string) => void}
+	 * @type {(input: string) => Promise<void>}
 	 */
 	export let onSubmit;
 
@@ -18,11 +18,15 @@
 	/** @type {ReturnType<typeof setTimeout>|null} */
 	let blurTimer = null;
 
-	function handleSubmit() {
+	async function handleSubmit() {
 		const trimmed = value.trim();
 		if (!trimmed || loading) return;
-		onSubmit(trimmed);
-		value = '';
+		try {
+			await onSubmit(trimmed);
+			value = '';
+		} catch {
+			// Value preserved for retry
+		}
 	}
 
 	/**
@@ -147,7 +151,7 @@
 	.input:focus {
 		outline: none;
 		border-color: var(--color-primary);
-		box-shadow: 0 0 0 2px rgba(79, 70, 229, 0.15);
+		box-shadow: 0 0 0 2px var(--focus-ring);
 	}
 
 	.submit-btn {