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
- Add bacon.toml for Rust development watching with keybindings
- Update Justfile to use bacon for dev watching
- Configure frontend to build to ./public for backend serving
- Improve Justfile organization with comments and better task separation
- Add dev-backend and dev-frontend tasks for separate workflows
- Minor formatting fix in backend/src/state.rs
Introduce AppError enum to replace panic-based error handling in executable loading and state management. Adds proper error propagation with descriptive error messages for missing executables, key patterns, and environment variables.
