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/app.html

@@ -33,6 +33,19 @@
 						(pref === 'system' && window.matchMedia('(prefers-color-scheme: dark)').matches);
 					if (dark) document.documentElement.classList.add('dark');
 				} catch (_) {}
+				// Colour-theme FOUC guard: re-apply the cached palette override before paint so
+				// a custom/preset theme doesn't flash the default gold. The value is a ready-to-
+				// inject `:root:root{…}` string from event-config-store (empty for the default
+				// theme); refreshed from /event once the app mounts.
+				try {
+					var css = localStorage.getItem('eventsnap_palette_css');
+					if (css) {
+						var s = document.createElement('style');
+						s.id = 'es-theme';
+						s.textContent = css;
+						document.head.appendChild(s);
+					}
+				} catch (_) {}
 			})();
 		</script>
 		%sveltekit.head%