add dev command with cargo watch on port 3000

- adds `just dev` command that runs cargo watch
- uses port 3000 to avoid conflicts with other local services
- watches src and templates directories for changes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[e-mail]>

justfile

···
 watch:
     watchexec -w src -w templates -r cargo run
 
+dev:
+    SERVER_PORT=3000 cargo watch -x run -w src -w templates
+
 deploy:
     fly deploy