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

sentry.server.config.ts

@@ -15,6 +15,8 @@ if (dsn) {
 			"AbortError",
 			"NEXT_REDIRECT",
 			"NEXT_NOT_FOUND",
+			"UnrecognizedActionError",
+			"Server Action.*was not found on the server",
 		],
 		beforeSend: redactSentryEvent,
 	});