feat: Playnite library sync plugin + exporter

A two-part Punktfunk plugin that syncs the Playnite library into the host
game library as the `playnite` provider:

- exporter/ — a minimal C# Playnite GenericPlugin ("Punktfunk Sync") that
  writes punktfunk-library.json on every library change. Reading Playnite's
  live-locked LiteDB from outside isn't robust, so this adapter runs inside
  Playnite. Builds net462 via reference assemblies; packaged as a .pext.
- src/ — the TS plugin (on @punktfunk/host, rom-manager shape): watches the
  export, embeds covers as data URLs, reconciles via
  PUT /library/provider/playnite, with a console-hosted web UI + standalone
  fallback + CLI. Launch = playnite://playnite/start/<id>, run by the host
  in the interactive session so Playnite performs the real launch.

Verified locally: backend tsc + 23 tests + biome clean, SPA build, C#
exporter build + .pext pack.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

exporter/PunktfunkSync.csproj

@@ -0,0 +1,40 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <!--
+    The Playnite side of the plugin: a GenericPlugin that writes `punktfunk-library.json` into its own
+    plugin-data dir on every library change, for the Punktfunk `playnite` plugin to read and reconcile.
+
+    Targets .NET Framework 4.6.2 (Playnite's runtime). `Microsoft.NETFramework.ReferenceAssemblies`
+    lets `dotnet build` compile net462 on Linux/macOS (no Windows/MSBuild needed), so CI builds it on
+    the same ubuntu runner as the TS side. PlayniteSDK is a COMPILE-only reference — Playnite provides
+    it at runtime — so `ExcludeAssets=runtime` keeps it out of the packaged `.pext`.
+  -->
+  <PropertyGroup>
+    <TargetFramework>net462</TargetFramework>
+    <AssemblyName>PunktfunkSync</AssemblyName>
+    <RootNamespace>PunktfunkSync</RootNamespace>
+    <LangVersion>latest</LangVersion>
+    <Nullable>disable</Nullable>
+    <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
+    <Deterministic>true</Deterministic>
+    <DebugType>none</DebugType>
+    <GenerateAssemblyInfo>true</GenerateAssemblyInfo>
+    <Version>0.1.0</Version>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="PlayniteSDK" Version="6.11.0">
+      <ExcludeAssets>runtime</ExcludeAssets>
+    </PackageReference>
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.3">
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+  </ItemGroup>
+
+  <ItemGroup>
+    <None Include="extension.yaml">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
+  </ItemGroup>
+
+</Project>