improvement(plugin): better architecture, introduced slot loading and module registry.

[xZepyx]

Describe your changes

Introduce ModuleRegistry, ModuleHost, SlotHost, SlotLoader, and PluginBus
to replace the flat LazyLoader/Contracts pattern in shell.qml.

• Add modules/hosts/{ModuleLoader,SlotHost,SlotLoader}.qml as qs.modules.hosts
• Add services/{ModuleRegistry,ModuleHost,PluginBus}.qml singletons
• Replace services/Contracts.qml with backward-compat shim over ModuleRegistry
• Collapse 10 raw LazyLoader blocks in shell.qml into single ModuleHost{}
• Fix loader resurrection bug: null _liveSource on deactivate, restore on
  activate, guaranteeing Loader sees a genuine source change every cycle
• Forward implicitHeight/implicitWidth from loaded item through SlotLoader
  so ColumnLayout can size slot children correctly
• Split SidebarRightContent into individual named slots: header, volumeSlider,
  brightnessSlider, networkToggle, flightModeToggle, bluetoothToggle,
  themeToggle, nightModeToggle, notifModal
• Add SidebarRightHeader.qml extracting header row from SidebarRightContent
• PluginBus.hookSlot/injectSlot API lets plugins replace, remove, or inject
  slots without holding direct references to any loader

Is it ready? Questions/feedback needed?

Almost, but not yet. Still needs a lot of stuff. I also need to implement slots for all modules.
Long story short it's not ready for merge rn.

[xZepyx]

Has errors and issues. Not ready for merge rn