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/DeferredPage.css

@@ -1 +0,0 @@
-.aksrt-deferred-page[data-v-e3307067]{display:flex;flex-direction:column;align-items:center;justify-content:center;min-height:400px;text-align:center;padding:40px 20px}.aksrt-deferred-page__icon[data-v-e3307067]{color:#c0c4cc;margin-bottom:20px}.aksrt-deferred-page__title[data-v-e3307067]{margin:0 0 10px;font-size:20px;font-weight:600;color:#1a1a2e}.aksrt-deferred-page__desc[data-v-e3307067]{margin:0 0 20px;font-size:14px;color:#8a909c;max-width:400px;line-height:1.6}.aksrt-deferred-page__meta[data-v-e3307067]{display:flex;gap:10px;margin-bottom:18px}.aksrt-deferred-page__depends[data-v-e3307067],.aksrt-deferred-page__reason[data-v-e3307067]{width:min(520px,100%);margin-bottom:18px;text-align:left}.aksrt-deferred-page__label[data-v-e3307067]{display:inline-block;margin-bottom:8px;font-size:12px;font-weight:600;color:#4b5563}.aksrt-deferred-page__depends-list[data-v-e3307067]{display:flex;flex-wrap:wrap;gap:8px}.aksrt-deferred-page__reason p[data-v-e3307067]{margin:0;color:#666;line-height:1.7}.aksrt-deferred-page__features[data-v-e3307067]{display:flex;flex-direction:column;gap:8px;margin-bottom:20px}.aksrt-deferred-page__feature[data-v-e3307067]{display:flex;align-items:center;gap:8px;font-size:13px;color:#666}.aksrt-deferred-page__feature[data-v-e3307067] i{color:#52c41a}