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.

src/GmRelay.Web/Components/Layout/NavMenu.razor

@@ -82,7 +82,7 @@
                     </button>
                 </form>
 
-                <div class="nav-version">v3.9.3</div>
+                <div class="nav-version">v3.9.4</div>
             </div>
         </Authorized>
         <NotAuthorized>