fix post-login blank page + rebrand login hero

- nginx: use alias for BASE_PATH fallback so static assets resolve
  correctly when deployed under a sub-path (fixes white page on K8s)
- base.html: remove dead template refs (chat, notes, projects) that
  caused html/template to fail silently with Content-Length: 0
- login hero: rebrand from "Chat Switchboard" to "Switchboard Core",
  update tagline and feature pills to reflect platform pivot

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

nginx.conf.template

@@ -90,8 +90,10 @@ server {
     }
 
     # Fallback: serve frontend shell (SPA)
+    # alias strips the BASE_PATH prefix so /test/css/app.css → /usr/share/nginx/html/css/app.css
     location ${BASE_PATH}/ {
-        try_files $uri $uri/ ${BASE_PATH}/index.html;
+        alias /usr/share/nginx/html/;
+        try_files $uri $uri/ /index.html;
     }
 
     # Security headers