firebase: app-vue target self-builds (predeploy) — fixes 'app-vue/dist does not exist'

The 'app' hosting target (socioprophet-builder site) pointed at
socioprophet-web/app-vue/dist but nothing built it, so 'firebase deploy' failed
with 'Directory ... does not exist'. Added a predeploy hook that installs + builds
app-vue (vue-tsc + vite build, verified: builds clean in ~0.6s) before deploy, so
the target is self-sufficient.

Author: mdheller

firebase.json

@@ -29,6 +29,10 @@
     {
       "target": "app",
       "public": "socioprophet-web/app-vue/dist",
+      "predeploy": [
+        "npm --prefix socioprophet-web/app-vue install",
+        "npm --prefix socioprophet-web/app-vue run build"
+      ],
       "ignore": [
         "firebase.json",
         "**/.*",