chore: remove standalone output mode, fix Sentry DSN validation, add dev CSP unsafe-inline for styles

- Remove output: 'standalone' from next.config.ts to allow normal 'next start'
- Allow empty SENTRY_DSN/NEXT_PUBLIC_SENTRY_DSN in env validation (zod)
- Add 'unsafe-inline' to style-src CSP only in development for Turbopack HMR
- Clear placeholder Sentry DSN values from .env

src/lib/csp.ts

@@ -20,6 +20,7 @@ export function buildContentSecurityPolicy(nonce: string): string {
 		"'self'",
 		`'nonce-${nonce}'`,
 		"https://fonts.googleapis.com",
+		...(isDev ? ["'unsafe-inline'"] : []),
 	].join(" ");
 
 	return [