Merge NothingLess improvements into Ambxst

[leriart]

This PR merges performance and feature improvements originally developed in the NothingLess fork back into Ambxst. All improvements are fully rebranded, with zero references to NothingLess remaining. The changes are purely additive and compatible with upstream.

Key improvements

• Removed mpvpaper, replaced with QtMultimedia + FFmpeg — Hardware-accelerated video wallpapers with lower overhead (GStreamer backend, VA-API support, configurable OpenGL/Vulkan rendering)
• Added ambxst-fps — Custom MangoHud-based FPS monitoring tool with real-time notch overlay display. Built-in MangoHud patches for rebuilding from source
• Added dynamic tasktray — Systray with smooth expansion animation, unified background, proper context menus, and perma-rounded corners
• Added monitors_writer.py — nwg-displays-style monitor configuration script with per-monitor positioning, hyprctl reload, and AxctlService integration
• Added Nothing Phone preset — Complete color theme (dark/light) + bar config with dot-matrix aesthetic, monochrome palette with subtle red accents, and Ndot font
• Added Nix support with KDE Kirigami — Quickshell package now includes kirigami, kirigami-addons, qqc2-desktop-style, and syntax-highlighting for better NixOS compatibility
• Optimized GLSL shaders — Reduced draw calls, shared GPU textures (GradientCache), and new interpolation shaders for smoother transitions
• Updated overview layout — Fill tiling, drag-drop windows, per-monitor positioning, unified input, live hyprctl refresh, and animation fixes

Housekeeping

• Replaced boot splash with original upstream animation
• Restored original upstream README, AGENTS.md, and logos
• Removed all fork branding and personal dev paths
• Cleaned up orphan assets

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Does this change any existing behavior?

Area	Before	After
Video wallpaper	mpv-based (external process)	QtMultimedia + FFmpeg (in-process, hardware-accelerated)
Rendering backend	Fixed OpenGL	Configurable: OpenGL (default) or Vulkan with threaded render loop
FPS monitoring	Not available	Built-in via ambxst-fps + notch metrics overlay
Systray	Static icons	Dynamic expansion with animation
Monitor config	Manual editing	monitors_writer.py with hyprctl live reload
Overview	Basic workspace view	Drag-drop, fill tiling, per-monitor, live refresh
Nix build	Minimal Quickshell	Quickshell with KDE Kirigami support