Share one secret display style and add the architecture atlas

The API key block and the workspace editor password each carried their
own copy of the same monospace, break-all, boxed styling. Both now use a
single secret-value class in base.css, so a new secret surface inherits
the presentation instead of duplicating it.

ARCHITECTURE.md is a structural overview of the application derived from
the source tree, the live schema and the imported app object, linked
from the README.

README.md

@@ -15,6 +15,8 @@ make test-headed    # same tests in visible browser
 
 Open `http://localhost:10500`.
 
+A measured structural overview of the whole application, fifteen Mermaid diagrams covering deployment, request pipeline, URL surface, data layer, AI plane, containers and the real-time plane, is in [ARCHITECTURE.md](ARCHITECTURE.md).
+
 PDF export (DeepSearch reports, via weasyprint) needs the Pango text stack installed at system level. The production image installs it; on a development host install it once:
 
 ```bash