Security hardening and fix all compile warnings

Security fixes:
- Validate theme background image URLs against whitelist pattern
- Add board permission check to Socket.IO join-board handler
- Disable raw HTML passthrough in markdown parser (XSS prevention)
- Add UUID validation on avatar and branding route params (path traversal)
- Add security headers (X-Frame-Options, X-Content-Type-Options, Referrer-Policy)
- Validate webhook URLs (HTTPS only, block internal addresses)
- Add import payload size limit (5MB)
- Remove server uptime from health endpoint
- Make seed password configurable via SEED_ADMIN_PASSWORD env var
- Patch DOMPurify dependency vulnerability via npm audit fix

Warning fixes:
- Convert $state(prop) to $effect.pre sync pattern (state_referenced_locally)
- Add for/id pairings to form labels (a11y_label_has_associated_control)
- Add svelte-ignore for intentional autofocus usage (a11y_autofocus)
- Add ARIA roles to interactive/overlay divs (a11y_no_static_element_interactions)
- Add aria-label to icon-only buttons (a11y_consider_explicit_label)
- Add keyboard handler alongside click events (a11y_click_events_have_key_events)
- Fix non-reactive fileInput declaration

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

src/hooks.server.ts

@@ -67,6 +67,11 @@ export const handle: Handle = async ({ event, resolve }) => {
 			: undefined
 	});
 
+	// Security headers
+	response.headers.set('X-Frame-Options', 'DENY');
+	response.headers.set('X-Content-Type-Options', 'nosniff');
+	response.headers.set('Referrer-Policy', 'strict-origin-when-cross-origin');
+
 	logger.info({
 		method: event.request.method,
 		path: event.url.pathname,