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.

README.md

@@ -7,7 +7,7 @@ Designed for sysadmins who want to wire up tools like **Zerto pre/post scripts**
 ## Quickstart
 
 1. **Download** the latest installer: <https://github.com/recklessop/webhook-server/releases/latest>
-2. **Run it.** UAC accept → next, next, finish. Adds a Start Menu entry, registers and starts the Windows Service.
+2. **Run it.** UAC accept → next, next, finish. Adds a Start Menu entry, registers and starts the Windows Service. The installer also downloads + installs the **.NET 8 runtimes** (ASP.NET Core + Desktop) if they're missing — fresh Windows Server installs need this.
 3. **Open Webhook Server** from the Start Menu (auto-elevates).
 4. **File → New endpoint**, configure a slug + script, save, hit the URL.