build: modernize project tooling

[lewxdev]

summary

• replace esrun with tsx for custom-server development and production startup
• migrate to ESLint 9 flat config for Next.js 16 and update the TypeScript, formatting, hook, and CSS tooling
• run Husky hooks through installed command shims, remove the unused Fly Dockerfile package, and adapt affected React code to the updated lint rules

why

this final slice removes the remaining vulnerable development dependency chain and aligns the project tooling with the upgraded framework.

stack

this is the final slice and builds on #45.

validation

• npm audit
• npm audit --omit=dev
• Fly review deployment

[drewradcliff]

🥞