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/env.ts

@@ -92,8 +92,8 @@ const schema = z
 		// Logging level.
 		LOG_LEVEL: z.enum(["debug", "info", "warn", "error"]).optional(),
 		// Optional Sentry — no-op when unset.
-		SENTRY_DSN: z.string().url().optional(),
-		NEXT_PUBLIC_SENTRY_DSN: z.string().url().optional(),
+		SENTRY_DSN: z.string().url().optional().or(z.literal("")),
+		NEXT_PUBLIC_SENTRY_DSN: z.string().url().optional().or(z.literal("")),
 		SENTRY_ORG: z.string().optional(),
 		SENTRY_PROJECT: z.string().optional(),
 		SENTRY_AUTH_TOKEN: z.string().optional(),