refactor: trim dependencies and use built-ins

[brookslybrand]

…environment variables
Trim dependencies and use built-ins

• Remove Luxon – Use Intl.DateTimeFormat in blog.server.ts for post dates, with timeZone: "America/New_York" so dates are consistent for all users.
• Bump isbot – Upgrade from ^4 to ^5 (for react-router).
• Replace tiny-invariant – Use Node node:assert in app/lib/blog.server.ts and app/routes/jam/schedule.server.ts.
• Remove dotenv / dotenv-cli – Use Node’s --env-file=.env in the preview script; use Vite’s loadEnv("test", ...) in vitest.config.mts for test env (.env, .env.test, etc.).
• Add .env.test – Test env is loaded via Vite’s loadEnv in test mode.