Migrate to Avalonia 12 and Plugin SDK v5

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.

docs/PLUGIN_SDK_V5_MIGRATION.md

@@ -0,0 +1,34 @@
+# Plugin SDK v5 Migration Guide
+
+Plugin SDK v5 is the Avalonia 12 compatibility baseline for LanMountainDesktop plugins.
+
+## What Changed
+
+- Rebuild plugins against `LanMountainDesktop.PluginSdk` `5.0.0`.
+- Set `plugin.json` `apiVersion` to `5.0.0`.
+- Target `net10.0` and use Avalonia `12.0.1` compatible UI dependencies.
+- Use `FluentAvaloniaUI` `3.0.0-preview1` and `FluentIcons.Avalonia` `2.1.325` when a plugin directly references those packages.
+
+## Compatibility
+
+SDK v5 is a binary breaking change because the SDK exposes Avalonia UI types such as `Control`, `UserControl`, and `SettingsPageBase`. Plugins built for SDK v4 must be rebuilt and republished for SDK v5.
+
+The host does not provide an Avalonia 11 / Avalonia 12 dual UI stack. The public extension entry points remain the same: custom settings pages still derive from `SettingsPageBase`, and desktop components still provide Avalonia controls through the existing registration APIs.
+
+## Minimal Package Update
+
+```xml
+<ItemGroup>
+  <PackageReference Include="LanMountainDesktop.PluginSdk" Version="5.0.0" />
+</ItemGroup>
+```
+
+```json
+{
+  "apiVersion": "5.0.0"
+}
+```
+
+## Validation
+
+After updating package versions and rebuilding the plugin, verify that the generated `.laapp` contains the rebuilt assembly, `plugin.json`, and `.deps.json` next to the plugin entry assembly.