- bump electron to match nixpkgs version (39.7.0)
- add bun2nix dev dependency and relevant postinstall script to update bun deps for nix
- added nix flake exposes a dev shell and a build package for nix

Add Nix flakes support

Clarify instructions for Nix development and building.

Updated contributing section and removed development instructions.

Added a reference to the react-electron-template for setup details.

Using an FHS env for the Nix shell was a quick and easy way to get
around Nix's non-FHS compliance but it caused sandboxing issues; namely,
the flatpak make target just silently fails to build. I decided to take
more time to just use `mkShell`, and patch some things around to get
electron and RPM to work nicely. Now, all electron-forge make targets
compile on Nix, without FHS! (At least, on my end)

```
✔ Making distributables
  ✔ Making a zip distributable for linux/x64 [40s]
  ✔ Making a deb distributable for linux/x64 [40s]
  ✔ Making a rpm distributable for linux/x64 [1m31s]
  ✔ Making a flatpak distributable for linux/x64 [4m8s]
```

I also confirmed that the zip and flatpak builds do work. For deb and
rpm, I had to use distrobox. The app does open, but I just couldn't get
the `linux-wallpaperengine` backend to compile due to some dynamic
library issues, but I'll assume the generated artifacts for the app are
correct.

Also full disclosure that the RPM workaround in here went through some
iterative prompting. I at least tried to leave the AI's thought process
behind it. The wrappers aren't that complicated but I'll have to admit
that I had to rely on a lot of help since I'm unfamiliar with how
Fedora's packaging works (and how forge does their magic with it in the
maker).

Do let me know if this plays better on your end (non-NixOS running Nix).

feat: tray and minimize [JAG-26]