chore: bump version 3.9.3 -> 3.9.4

Test-only patch release. Sync the four canonical version sources:
- Directory.Build.props
- compose.yaml (bot, discord, web image tags)
- .gitea/workflows/deploy.yml (VERSION env)
- src/GmRelay.Web/Components/Layout/NavMenu.razor (visible nav-version)

The new NavMenu_ShouldExposeCurrentProjectVersion test reads the
version from Directory.Build.props, so this bump does NOT need a
hand-edited test literal — verified locally that the test passes
on 3.9.4 with no manual changes.

Directory.Build.props

@@ -1,6 +1,6 @@
 <Project>
   <PropertyGroup>
-    <Version>3.9.3</Version>
+    <Version>3.9.4</Version>
     <TargetFramework>net10.0</TargetFramework>
     <LangVersion>preview</LangVersion>
     <Nullable>enable</Nullable>