Add monitor detection module using RandR to detect connected monitors,
their positions, and primary display. UI elements now center on the
primary monitor instead of the full virtual screen.

Changes:
- Add monitors.rs with MonitorConfig and Monitor structs
- Add randr feature to x11rb dependency
- Update LoginForm, UserList, PowerButtons to accept center coordinates
- Add root() accessor to GreeterWindow for RandR queries
- Add sprint documentation for sprints 5-8

- Avatar loading from ~/.face, AccountsService, and other standard paths
- Circular avatar rendering with RGBA to BGRA conversion
- Fallback to colored initials when no avatar found
- Horizontal user list above login form with hover effects
- Click user to populate username and focus password field
- Selection highlight with blue ring

- Custom Cairo-drawn icons (power, reboot, suspend, chevron, checkmark)
- Power buttons with color-coded hover highlights
- Session selector dropdown with list from daemon
- Mouse tracking for hover states
- IPC integration for power actions

- Read garbg config and playlist state for wallpaper resolution
- Greeter config with visual settings (blur, brightness, corner radius)
- Fade-out transition on successful login
- Relaxed socket permissions for development (0666)

- X11 window with fullscreen override redirect
- Cairo rendering surface for drawing
- Background loader with blur and brightness adjustment
- Login form widget with username/password fields
- Keyboard input handling with X11 keycodes
- IPC integration with gardmd for authentication