feat(theme+comments): runtime colour theme and COMMENTS_ENABLED switch

Colour theme is configurable at runtime from two seed colours (brand + accent);
neutrals stay fixed for contrast safety. Tailwind v4 var()-based tokens let a
:root:root override recolour everything with no rebuild; the 50->950 ramps are
derived via a color-mix ladder. Config lives in the DB config table (admin UI:
Config > Farbschema, presets + custom pickers + live preview), served on the public
/event endpoint with env defaults (THEME_PRESET/PRIMARY/ACCENT), propagated live via
event-updated SSE, and cached in localStorage for a no-flash boot. The keepsake export
mirrors the same ladder in Rust so offline archives match the event theme.

COMMENTS_ENABLED (env, default true) is a boot-time kill-switch: the backend rejects
new comments with 403 and the frontend hides the comment button (feed card) and
panel/composer (lightbox). Existing comments stay in the DB, hidden, and return when
re-enabled.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/event-config-store.ts

@@ -0,0 +1,60 @@
+/**
+ * Public event presentation config: the colour theme and the comments feature flag,
+ * served unauthenticated from `GET /api/v1/event`. Loaded once on boot (and refreshed
+ * on the `event-updated` SSE) so the theme applies before the user has even joined and
+ * the comment UI reflects the instance setting.
+ *
+ * The resolved theme CSS is cached in localStorage so the boot script in app.html can
+ * paint the right colours before the JS bundle loads (no flash of the default palette).
+ */
+import { writable } from 'svelte/store';
+import { buildPaletteCss, applyPaletteCss, type ThemeConfig } from './theme/palette';
+
+export const PALETTE_CACHE_KEY = 'eventsnap_palette_css';
+
+export type EventConfig = {
+	name: string;
+	slug: string;
+	commentsEnabled: boolean;
+	theme: ThemeConfig;
+};
+
+export const eventConfig = writable<EventConfig | null>(null);
+/** Convenience flag for the comment UI. Optimistic `true` until /event resolves. */
+export const commentsEnabled = writable<boolean>(true);
+
+/** Fetch /event, apply the theme, and cache the resolved CSS for the next boot. */
+export async function loadEventConfig(): Promise<void> {
+	try {
+		const res = await fetch('/api/v1/event');
+		if (!res.ok) return;
+		const b = await res.json();
+		const theme: ThemeConfig = {
+			preset: b.theme_preset ?? 'champagne-gold',
+			primary: b.theme_primary ?? '#8a6a2b',
+			accent: b.theme_accent ?? '#8a6a2b'
+		};
+		eventConfig.set({
+			name: b.name,
+			slug: b.slug,
+			commentsEnabled: b.comments_enabled ?? true,
+			theme
+		});
+		commentsEnabled.set(b.comments_enabled ?? true);
+
+		const css = buildPaletteCss(theme);
+		applyPaletteCss(css);
+		try {
+			localStorage.setItem(PALETTE_CACHE_KEY, css);
+		} catch {
+			/* private mode / quota — the theme still applied for this session */
+		}
+	} catch {
+		/* offline / server down — keep whatever the boot script already painted */
+	}
+}
+
+/** Apply a theme immediately without persisting — used by the admin live preview. */
+export function previewTheme(theme: ThemeConfig): void {
+	applyPaletteCss(buildPaletteCss(theme));
+}