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

next.config.ts

@@ -32,9 +32,6 @@ const nextConfig: NextConfig = {
 	// Disable Next.js telemetry and browser sourcemaps in production
 	productionBrowserSourceMaps: false,
 
-	// Standalone output for smaller, faster Docker deployments and cold starts
-	output: "standalone",
-
 	experimental: {
 		useTypeScriptCli: true,
 	},