Auto-install .NET 8 runtimes if missing

A fresh Windows Server install has neither ASP.NET Core 8 nor .NET
Desktop 8, so the Service refuses to start and the GUI fails to
launch — the symptoms link to aka.ms/dotnet-core-applaunch but it's
not obvious from the GUI itself what's wrong.

Setup now:
- Detects each runtime by looking for an 8.x folder under
  %ProgramFiles%\dotnet\shared\Microsoft.{AspNetCore,WindowsDesktop}.App
- Uses Inno Setup's built-in TDownloadWizardPage to fetch any missing
  runtime from the aka.ms redirect and runs `/install /quiet /norestart`
- Treats Microsoft's 1638 / 3010 / 1641 exit codes as success

If the box has no internet, the prereq install fails with a clear
message and a "continue anyway?" prompt. README + installation docs
list the manual download URLs and `dotnet --list-runtimes` check;
troubleshooting has a section pointing at the same fix when the
service won't start after install.

docs/troubleshooting.md

@@ -38,6 +38,28 @@ You launched the GUI without elevation. The admin pipe ACL is `SYSTEM` + `Admini
 
 **Fix in v0.1.0**: right-click the Start Menu shortcut → **Run as administrator**, or upgrade.
 
+### Service won't start after install / GUI says "Disconnected" with no obvious error
+
+If `Get-Service WebhookServer` shows it stopped and `Start-Service WebhookServer` fails, or the GUI itself won't even launch, you're probably missing a .NET 8 runtime. The v0.1.4+ installer auto-fetches them, but a clean Windows Server box might still hit this if the install was offline or used an older installer.
+
+Check what's installed:
+
+```powershell
+dotnet --list-runtimes
+```
+
+You need both:
+
+- `Microsoft.AspNetCore.App 8.x.y` — for the Service
+- `Microsoft.WindowsDesktop.App 8.x.y` — for the GUI
+
+If either is missing, install from:
+
+- ASP.NET Core 8 Runtime — <https://aka.ms/dotnet/8.0/aspnetcore-runtime-win-x64.exe>
+- .NET Desktop Runtime 8 — <https://aka.ms/dotnet/8.0/windowsdesktop-runtime-win-x64.exe>
+
+Re-run with `/install /quiet /norestart` for unattended installs. Then `Start-Service WebhookServer`.
+
 ### "Connection refused" hitting the hook URL
 
 Three possibilities, in order of probability: