Attempt/nix support

[Brainitech]

Overview

This PR introduces full declarative support for NixOS. Instead of relying on imperative package management, Brain Shell can now be installed natively as a Nix flake. The flake handles all Qt6, QuickShell, and system dependencies immutably, while a new distro-specific bash script wires up the mutable user configs and Hyprland autostarts.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (causes existing functionality to not work as expected)

Testing Checklist

• I have tested these changes locally and on a fresh VM install.
• I have tested the QML UI on my primary resolution.
• (If applicable) I have tested UI changes on secondary monitors or different resolutions.
• (If applicable) I have verified my bash scripts run cleanly without syntax errors.

Description

Arch Linux remains fully supported and untouched via the existing install-arch.sh. For NixOS, the architecture is split to respect Nix conventions:

• System Dependencies: Managed completely via flake.nix (pulls QuickShell, Qt6, Wayland tools, fonts, and system services into environment.systemPackages).

• User State: Managed via install-nix.sh to handle ~/.config generation, cache initialization, and safely appending autostarts to the active Hyprland config.

Changes

• Added flake.nix & flake.lock: Configured inputs, Qt platform variables, and the system
• Fixed Font Rendering: Registered nerd-fonts.jetbrains-mono and nerd-fonts.symbols-only explicitly in the flake via fonts.packages to ensure icon rendering works out of the.
• Created install-nix.sh: Handles Hyprland autostart injection, config file creation, and cache dir.
• Ported Keybind Checker: Carried over the live Python-based keybind conflict checker from the Arch script into the NixOS script so it intelligently checks the active Hyprland.
  Updated README: Added clean, 4-step NixOS setup instructions for end users.

Related Issues

Closes #58 , Closes #59, Resolves #70