feat: add niri animation presets support

[cnvuls]

Summary

Adds support for custom animation presets in Settings, Compositor, Animations.

Users can now easily pick any custom .kdl animation preset from ~/.config/niri/animations/ (e.g. glide.kdl, smoke.kdl, bloom.kdl) or revert to default (config.d/60-animations.kdl) via a dropdown (StyledComboBox) in iNiR Settings.

Changes:

• scripts/niri-config.py:
• get-animation-presets: lists available *.kdl preset files in ~/.config/niri/animations/ and detects the current active include in config.kdl.
• set-animation-preset: surgically updates the include directive in ~/.config/niri/config.kdl.
• modules/settings/NiriConfig.qml:
• Added animationPresetsData property, animationPresetsProcess and setAnimationPresetProcess.
• Added ContentSubsection with StyledComboBox in the Animations card section.

Testing

• inir restart && inir logs — no errors
• Tested selecting custom animation presets from UI and verified config.kdl includes update properly
• Both panel families checked (if shared code changed)

Notes

Enhances Niri customization by making animation presets switchable directly from the GUI.