Add ui-structure.md — full UI inventory and widget taxonomy

Covers all 7 nav sections with layout descriptions, all 11 dialog types,
recurring widget patterns (split pane, tabbed view, table, gallery, card,
field editor, modal dialog, non-modal overlay, full-page sub-view, canvas,
lightbox) with every instance listed, complete template inventory (41 entries),
and a primitive taxonomy for a future higher-level UI representation.
Linked from CLAUDE.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -378,6 +378,14 @@ git config user.email 'mikaels.claude.agent@efforting.tech'
 
 ---
 
+## UI structure
+
+See [`ui-structure.md`](ui-structure.md) for the full inventory of sections,
+widget patterns, dialogs, templates, and a taxonomy of primitives relevant to
+designing a higher-level UI representation.
+
+---
+
 ## Known limitations / planned rewrite notes
 
 See [`future-plans.md`](future-plans.md) for full detail. Key points relevant