GUI subsystem: -H=windowsgui + AttachConsole, MB_TOPMOST on test window

drover.exe is now a GUI subsystem binary:
  - Double-click no longer flashes a console window — a clean
    smoke-test message box opens immediately.
  - When run from cmd / PowerShell, AttachConsole reattaches stdout
    and stderr to the parent terminal so '--version', 'check', etc.
    still print as expected.
  - MB_TOPMOST flag added to MessageBox so the window can't be
    obscured by other windows on launch (this was the actual cause
    of "I clicked but nothing happened" reports).

Verified locally: built with GOOS=windows GOARCH=amd64 -H=windowsgui;
running drover-gui.exe --version prints to PowerShell, drover-gui.exe
gui shows the message box on top of the active window.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -92,7 +92,11 @@ jobs:
           SHORT_SHA="${GITHUB_SHA:0:7}"
           BUILD_DATE="$(date -u +%Y-%m-%d)"
           mkdir -p bin
-          go build -trimpath -ldflags="-s -w \
+          # -H=windowsgui = subsystem WINDOWS, double-click no longer
+          # flashes a console window. main.go calls AttachConsole on
+          # startup so CLI invocations from cmd/PowerShell still print
+          # to the parent terminal.
+          go build -trimpath -ldflags="-s -w -H=windowsgui \
             -X main.Version=dev-${SHORT_SHA} \
             -X main.Commit=${SHORT_SHA} \
             -X main.BuildDate=${BUILD_DATE}" \