feat: implement Story 1.1 — create and view diagrams

Add diagram/project DB schema, CRUD API, dashboard pages with grid/card/
empty state, create dialog with type selector, editor placeholder, and
26 schema validation tests. Includes code review fixes: soft-delete
filter on GET /:id, error handling, keyboard accessibility, type-safe
API response types, and error states on pages.

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

.gitignore

@@ -67,3 +67,6 @@ dist/
 
 # Auto Claude data directory
 .auto-claude/
+
+# Reference repos (not part of this project)
+_reference/