feat(tool): cross-platform server switcher (Prospect.Client.Config)

New console utility (net8.0, Linux/Proton + Windows) to point a client at
a server by writing backend.txt — the single value the Prospect agent
reads. Presets for PROD/PREPROD LAN and PROD internet, plus custom URL,
certificate import (native on Windows, guided with Proton-prefix
detection on Linux) and game launch. Interactive menu + scriptable CLI
(--folder/--set). Added to the solution; not built by the server CI.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/Prospect.Client.Config/Prospect.Client.Config.csproj

@@ -0,0 +1,15 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <OutputType>Exe</OutputType>
+        <TargetFramework>net8.0</TargetFramework>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <Nullable>enable</Nullable>
+        <AssemblyName>ProspectServerSwitcher</AssemblyName>
+        <RootNamespace>Prospect.Client.Config</RootNamespace>
+        <InvariantGlobalization>true</InvariantGlobalization>
+        <!-- Cross-platform client tool (Linux/Proton + Windows). Never built by the Linux server CI. -->
+        <Configurations>Debug;Release;Season 3 Release;Season 2 Release;Season 2 Debug;Season 3 Debug</Configurations>
+    </PropertyGroup>
+
+</Project>