Upgrade project to the Avalonia 12 baseline and Plugin SDK v5: centralize Avalonia packages, remove legacy WebView.Avalonia usage (use NativeWebView/WebView2 EnvironmentRequested), and update Fluent/Material icon/package usages. Bump multiple package/project versions to 5.0.0 and Avalonia 12.0.1, update plugin template and README/docs to SDK v5, and add PLUGIN_SDK_V5_MIGRATION.md.
Also fix runtime/behavior bugs: make DataLocationResolver use a fixed bootstrap launcher data path and avoid recursive ResolveDataRoot; add legacy-state handling and extraction in OobeStateService; and update component settings tests to reflect migrated storage (DB/backup) and reset cache for test reloads. Various csproj, tests, and docs updated to reflect the migration and ensure build/test compatibility.
Introduce a new LanMountainDesktop.Shared.IPC project implementing a public IPC host and client (LanMountainDesktopIpcClient, PublicIpcHostService), IPC constants and routed notify IDs, DTOs and DI helpers for registering public services. Update Plugin SDK to allow plugins to contribute public IPC services and registrations, add related descriptors/records and extension helpers. Migrate Launcher/App to use the new public IPC for startup/loading notifications and wiring (including TryConnect helper), switch LoadingStateReporter to use the external notification publisher, and add host-side public services (app info, shell control, plugin catalog). Include integration tests and spec/checklist/docs for the external IPC public API.
