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 .

frontend/src/App.svelte

@@ -0,0 +1,10 @@
+<script>
+  // Stub — will be replaced with actual UI
+</script>
+
+<div style="display:flex;align-items:center;justify-content:center;height:100vh;background:#13131f;color:#e4e4ef;font-family:sans-serif">
+  <div style="text-align:center">
+    <h1 style="font-size:28px;margin-bottom:8px">&#9874; Верстак</h1>
+    <p style="color:#8888a4">Wails desktop app — under construction</p>
+  </div>
+</div>

frontend/src/main.js

@@ -0,0 +1,4 @@
+import { mount } from 'svelte'
+import App from './App.svelte'
+
+mount(App, { target: document.getElementById('app') })

frontend/src/vite-env.d.ts

@@ -0,0 +1,2 @@
+/// <reference types="svelte" />
+/// <reference types="vite/client" />