Remove BepInEx client patcher (DorkNet.ClientPatch)

MelonLoader (DorkNet.ClientMod) is the sole client patcher now:

- delete the DorkNet.ClientPatch project + tools/install-plugin.ps1
- drop the project from DorkNet.sln
- README / deploy docs: present DorkNet.ClientMod (install-melon.ps1)
  as the patcher instead of the BepInEx plugin
- drop the dead install-plugin.ps1 reference from Program.cs's JWT
  error and install-melon.ps1's DRM-fallback hint
- tidy stray 'BepInEx port' comments

Legacy-cleanup paths that REMOVE an old BepInEx install
(install-legacy-client.ps1, build-patcher-zip.ps1's old_plugin_paths)
are kept intentionally.

DorkNet.Server/Program.cs

@@ -218,8 +218,7 @@ var jwtSecret = Environment.GetEnvironmentVariable("DORKNET_JWT_SECRET")
     ?? builder.Configuration["Jwt:Secret"]
     ?? throw new InvalidOperationException(
         "JWT secret not configured. Set the DORKNET_JWT_SECRET env var or " +
-        "add `Jwt:Secret` to appsettings.Local.json (the install-plugin.ps1 " +
-        "script generates one automatically on same-machine setup).");
+        "add `Jwt:Secret` to appsettings.Local.json.");
 
 builder.Services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
     .AddJwtBearer(opt =>