Add code field to archive content edit form and IDB

- Expose archive_content.code in edit form (trusted + edit_mode only)
- Add code to properties_to_save so it persists on every API load/save
- Add code field + index to Archive_Content Dexie interface (schema v2)
- Minor: center "Add" button rows in archive and content list components

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

src/lib/ae_archives/ae_archives__archive_content.ts

@@ -299,6 +299,7 @@ export const properties_to_save = [
     'archive_id',
     'archive_content_type',
     'name',
+    'code',
     'description',
     'content_html',
     'content_json',