vue-router was accidentally left over from an earlier attempt to embed
admin features in the main client. It broke hash-based session routing
and rendered app-main empty. Revert to direct component rendering in
App.vue with native hashchange handling.

- Remove vue-router dependency and src/router/index.js
- Revert main.js to createApp without router
- App.vue: render WelcomeScreen/ChatArea directly, restore hashchange handler
- chat.js: revert hash format to raw session id
- api/index.js: keep 401 → authStore.user = null (login overlay fix)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>