fix(desktop): default shareable URL to localhost web in dev

The renderer's navigation adapter fell back to https://multica.ai when
VITE_APP_URL was unset (i.e. desktop dev builds), so "Copy link" in a dev
build produced a production URL instead of one pointing at the running
dev web frontend. Match the fallback used by pages/login.tsx
(http://localhost:3000) so dev links stay on the dev host.

apps/desktop/src/renderer/src/platform/navigation.tsx

@@ -15,10 +15,11 @@ import {
 } from "@/stores/tab-store";
 import { useWindowOverlayStore } from "@/stores/window-overlay-store";
 
-// Public web app URL — injected at build time via .env.production. Falls
-// back to the production host for dev builds so "Copy link" yields a URL
-// that actually points somewhere a teammate can open.
-const APP_URL = import.meta.env.VITE_APP_URL || "https://multica.ai";
+// Public web app URL — injected at build time via .env.production. In dev
+// (no VITE_APP_URL set) falls back to the local web dev server so "Copy
+// link" in a dev build yields a URL that points at the running dev
+// frontend, not the prod host. Matches the fallback used in pages/login.tsx.
+const APP_URL = import.meta.env.VITE_APP_URL || "http://localhost:3000";
 
 /**
  * Extract the leading workspace slug from a path, or null if the path isn't