docs(web-ui): move css-vars.md into web-ui/

The CSS theme-variable reference documents the dashboard/agent web-UI
stylesheets, so it belongs under docs/web-ui/ alongside shape/dashboard/
agent. Wire it into the web-ui.md index (split list + reading path) and
the CLAUDE.md file map. Closes #1405.

docs/web-ui.md

@@ -19,6 +19,9 @@ This doc has been split for readability. Pick the section you need:
 - **[Per-agent page](web-ui/agent.md)** — header, main terminal,
   composer, side panel + inbox, live view, slash commands,
   per-agent endpoints, stats page.
+- **[CSS theme variables](web-ui/css-vars.md)** — the Catppuccin
+  Mocha custom properties declared once in `base.css` and the rule
+  that per-page stylesheets reference (never redeclare) them.
 
 ## Reading paths
 
@@ -37,3 +40,6 @@ This doc has been split for readability. Pick the section you need:
   [`web-ui/agent.md`](web-ui/agent.md) (Terminal-embedded prompt).
 - **"What are the per-agent HTTP endpoints?"** →
   [`web-ui/agent.md`](web-ui/agent.md) (Per-agent endpoints).
+- **"Which CSS variable do I use / where are colours defined?"** →
+  [`web-ui/css-vars.md`](web-ui/css-vars.md) (Palette, single-source
+  `base.css` rule).