gui: add Wails v3 desktop app skeleton

- Go upgraded to 1.25.10 (required by Wails v3)
- Wails v3 installed (alpha.96)
- Frontend: Svelte+Vite scaffold in frontend/
- guimain.go: Wails GUI entry point (compiled with -tags gui)
- wails_service.go: stub service for Wails bindings
- Verstak desktop binary builds: go build -tags gui -o verstak-gui .
- CLI (./cmd/verstak/) unaffected
- Legacy HTTP GUI (internal/gui/) preserved as prototype
- Build: 24MB ELF binary with GTK4/WebKit2GTK-6

Build commands:
  CLI:  go build -o verstak ./cmd/verstak/
  GUI:  cd frontend && npm run build && go build -tags gui -o verstak-gui .

build/linux/desktop

@@ -0,0 +1,13 @@
+[Desktop Entry]
+Version=1.0
+Name=My Product
+Comment=A verstak application
+# The Exec line includes %u to pass the URL to the application
+Exec=/usr/local/bin/verstak %u
+Terminal=false
+Type=Application
+Icon=verstak
+Categories=Utility;
+StartupWMClass=verstak
+
+