feat(dev): add Overmind-based development workflow

Add Procfile.dev for parallel backend/frontend development with
Overmind, replacing the sequential build approach. The new `just dev`
recipe starts both servers simultaneously with log multiplexing.

Changes:
- Update `just dev` to use Overmind
- Add `build-demo` recipe with optional Windows cross-compilation
- Add `smoke` recipe to verify development setup
- Improve logging in backend with bind address and dev mode indicator
- Allow missing Windows executable in debug builds

.gitignore

@@ -3,3 +3,4 @@
 demo-*
 /public
 .env
+.overmind.sock