style(data-stores): add icon to Edit button; update style guidelines

- Edit button now has SquarePen icon + text per button UX standard
- Style guidelines: extract button icon+text rule into its own section 8
  (was buried in Accessibility); renumber Accessibility to section 9;
  add code examples and context table for the rule

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

src/routes/core/data_stores/+page.svelte

@@ -15,6 +15,7 @@ import {
     LoaderCircle,
     Pencil,
     Plus,
+    SquarePen,
     RefreshCw,
     Search,
     X
@@ -596,8 +597,9 @@ function content_preview(ds: ae_DataStore): string {
                                     <button
                                         type="button"
                                         class="btn btn-sm preset-tonal-primary text-xs"
-                                        onclick={() => open_edit(ds)}>
-                                        Edit
+                                        onclick={() => open_edit(ds)}
+                                        title="Edit full record">
+                                        <SquarePen size={12} class="mr-1" /> Edit
                                     </button>
                                 </td>
                             </tr>