feat(core): add Data Stores management page at /core/data_stores

Full CRUD for all data_store records: search by code, account_id,
for_type, for_id; create/edit/delete via modal with type-aware content
editor (CodeMirror / TipTap / textarea). Wired into core nav and
dashboard. for_id shown read-only on edit (DB integer FK constraint).

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

src/routes/core/+layout.svelte

@@ -4,6 +4,7 @@ import { goto } from '$app/navigation';
 import { ae_loc, ae_sess, ae_api, slct } from '$lib/stores/ae_stores';
 import {
     Building,
+    Database,
     Globe,
     History,
     LayoutDashboard,
@@ -71,6 +72,9 @@ onMount(() => {
             <a href="/core/lookups" class="btn btn-sm preset-tonal-surface">
                 <List size={14} class="mr-1" /> Lookups
             </a>
+            <a href="/core/data_stores" class="btn btn-sm preset-tonal-surface">
+                <Database size={14} class="mr-1" /> Data Stores
+            </a>
         </nav>
 
         <section class="main_content grow px-1 pb-28 md:px-2">