feat: add optional extension hooks for private overlays

Three small, backwards-compatible hooks let anyone with private features
(velxio.dev's analytics, custom integrations, paid tiers, …) layer them
on top of the open-source build without forking files.

Backend (app/main.py):
- After standard router registration, try-import an optional `app.pro`
  module exposing `register_pro(app)`. ImportError is silently swallowed
  (the OSS image doesn't ship `app.pro`, so this is a no-op there).

Frontend:
- EditorToolbar: new optional `rightSlot` prop renders extra elements
  after the built-in right-group buttons (mirrors the existing
  `centerSlot` pattern).
- main.tsx: dynamic `import('@pro/index')` gated by VITE_PRO_BUILD env.
  When unset (OSS build), the branch is dead-code-eliminated and no pro
  chunk is emitted.
- vite.config.ts: `@pro` alias resolves to `src/__pro_stub__/` by default.
  Private builds set `VITE_PRO_BUILD=true` and `PRO_OVERLAY_PATH=<path>`
  to point at their real overlay tree.
- src/__pro_stub__/index.ts: 1-line no-op `mountPro` so TypeScript and
  Vite resolvers stay happy in OSS builds.

Verified: `npm run build:docker` succeeds; `npm test` passes 1161/1162;
the OSS bundle (43 MB) contains zero references to `__pro_stub__`,
`@pro`, or `pro/index` (verified via `grep dist/`).

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

backend/app/main.py

@@ -126,6 +126,17 @@ app.include_router(simulation.router, prefix="/api/simulation", tags=["simulatio
 from app.api.routes import iot_gateway
 app.include_router(iot_gateway.router, prefix="/api/gateway", tags=["iot-gateway"])
 
+# Optional pro extension. The `app.pro` package only exists in private builds
+# (overlaid at Docker build time by an external repo) — its absence in the
+# open-source image is expected and silently ignored. Anyone with private
+# extensions can drop a package at `backend/app/pro/` exposing
+# `register_pro(app)` and have it auto-loaded here without further edits.
+try:
+    from app.pro import register_pro  # type: ignore[import-not-found]
+    register_pro(app)
+except ImportError:
+    pass
+
 @app.get("/")
 def root():
     return {

frontend/src/__pro_stub__/index.ts

@@ -0,0 +1,7 @@
+// Open-source no-op stub for the @pro alias (see vite.config.ts).
+// Private overlays (e.g. velxio-prod) replace this at build time by setting
+// VITE_PRO_BUILD=true and PRO_OVERLAY_PATH to their real pro/frontend/src/pro
+// directory. The dynamic `import('@pro/index')` in main.tsx is gated by
+// VITE_PRO_BUILD, so this stub is unreachable in OSS builds — it exists only
+// to keep the TypeScript and Vite resolvers happy.
+export const mountPro = () => {};

frontend/src/components/editor/EditorToolbar.tsx

@@ -33,6 +33,12 @@ interface EditorToolbarProps {
    * regardless of how narrow the editor pane gets.
    */
   centerSlot?: React.ReactNode;
+  /**
+   * Optional extra elements rendered after the built-in right-group buttons
+   * (Libraries / Import-Export / Output Console). Used by private overlays
+   * to add deployment-specific actions without forking the toolbar.
+   */
+  rightSlot?: React.ReactNode;
 }
 
 const BOARD_PILL_ICON: Record<BoardKind, string> = {
@@ -63,6 +69,7 @@ export const EditorToolbar = ({
   compileLogs: _compileLogs,
   setCompileLogs,
   centerSlot,
+  rightSlot,
 }: EditorToolbarProps) => {
   const { files, codeChangedSinceLastCompile, markCompiled } = useEditorStore();
   const {
@@ -973,6 +980,7 @@ export const EditorToolbar = ({
                 <line x1="12" y1="19" x2="20" y2="19" />
               </svg>
             </button>
+            {rightSlot}
           </div>
         </div>
       </div>

frontend/src/main.tsx

@@ -15,3 +15,13 @@ import './components/velxio-components/EPaperElement';
 import App from './App.tsx';
 
 createRoot(document.getElementById('root')!).render(<App />);
+
+// Optional pro overlay. The `@pro` import resolves to a no-op stub in the
+// open-source build (see vite.config.ts) and to the real overlay only when
+// VITE_PRO_BUILD=true at build time. The dynamic import keeps the pro chunk
+// out of the OSS bundle entirely (Vite tree-shakes the never-taken branch).
+if (import.meta.env.VITE_PRO_BUILD) {
+  import('@pro/index')
+    .then((m) => m.mountPro?.())
+    .catch((err) => console.warn('[pro] failed to load overlay:', err));
+}

frontend/vite.config.ts

@@ -1,11 +1,27 @@
 import { defineConfig } from 'vite'
 import react from '@vitejs/plugin-react'
+import path from 'path'
 
 // https://vite.dev/config/
 // avr8js / rp2040js / @wokwi/elements are resolved from npm via package.json.
 // (The third-party/ clones are reference-only — keep them updated for credits.)
+//
+// The `@pro` alias resolves to a no-op stub by default. Private overlays
+// (e.g. velxio-prod) set VITE_PRO_BUILD=true and PRO_OVERLAY_PATH at build
+// time to point at their actual pro source tree. See README's "Pro overlay"
+// section.
+const proOverlayPath =
+  process.env.VITE_PRO_BUILD && process.env.PRO_OVERLAY_PATH
+    ? path.resolve(process.env.PRO_OVERLAY_PATH)
+    : path.resolve(__dirname, 'src/__pro_stub__')
+
 export default defineConfig({
   plugins: [react()],
+  resolve: {
+    alias: {
+      '@pro': proOverlayPath,
+    },
+  },
   server: {
     proxy: {
       '/api': {