docs(admin): document active item and breadcrumb logic for nav registration

- add note in DEV.md explaining basePath auto-deduction and breadcrumb slug convention
- update README.md to document new `basePath` param in `registerNavItem` and detail active item/breadcrumb behavior
- update navigation file listing in README.md to include new exported helpers
- implement `getNavItemBasePath` and `findActiveNavContext` in navigation.js
- use `basePath` in AdminSidebar to determine active item via longest-prefix match
- use `basePath` in AdminTop to build breadcrumb with section, item, and action labels
- expose new navigation helpers from admin index.js and registry.js

src/features/admin/registry.js

@@ -21,7 +21,7 @@ if (!globalThis[REGISTRY_KEY]) {
   globalThis[REGISTRY_KEY] = {
     widgetFetchers:   new Map(), // id -> async () => data
     widgetComponents: new Map(), // id -> { Component, order, permission }
-    navItems:         new Map(), // id -> { id, label, icon, href, order, sectionId, position, permission }
+    navItems:         new Map(), // id -> { id, label, icon, href, basePath, order, sectionId, position, permission }
     navSections:      new Map(), // id -> { id, title, icon, order }
     pages:            new Map(), // slug -> { slug, Component, title?, breadcrumbLabel? }
   };
@@ -66,8 +66,8 @@ export function registerNavSection({ id, title, icon, order = 0 }) {
   navSections.set(id, { id, title, icon, order });
 }
 
-export function registerNavItem({ id, label, icon, href, order = 0, sectionId = 'main', position, permission }) {
-  navItems.set(id, { id, label, icon, href, order, sectionId, position, permission });
+export function registerNavItem({ id, label, icon, href, basePath, order = 0, sectionId = 'main', position, permission }) {
+  navItems.set(id, { id, label, icon, href, basePath, order, sectionId, position, permission });
 }
 
 export function getNavSections() {