Restructure Core management into linkable routes and add common layout navigation

- Renamed person route to people for consistency
- Refactored /core dashboard to use standard <a> links
- Added common management <nav> to core layout
- Replaced goto() with standard links across all list and detail pages
- Fixed 500 error caused by broken import path and strict layout data

src/routes/core/+page.ts

@@ -0,0 +1,8 @@
+import type { PageLoad } from './$types';
+
+export const load: PageLoad = async ({ parent }) => {
+    const parentData = await parent();
+    return {
+        account_id: parentData.account_id
+    };
+};