chore(admin): stop tracking SPA build; add native product meta box

Remove assets/admin/build from version control (local build only going
forward). Add AKSRT_Product_Meta_Box so product price/stock/shipping/after-
sale fields can be edited on the native aksrt_product edit screen without
the SPA product drawer.

assets/admin/build/AttachmentPermissions.css

@@ -1 +0,0 @@
-.aksrt-attach__summary[data-v-a728445f]{display:flex;align-items:center;gap:10px;padding:12px;border-radius:6px;background:#fafafa}.aksrt-attach__summary strong[data-v-a728445f]{display:block;font-size:13px;color:#1a1a2e}.aksrt-attach__summary p[data-v-a728445f]{margin:2px 0 0;font-size:12px;color:#8a909c}