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/PageHeader.css

@@ -1 +0,0 @@
-.aksrt-page-header[data-v-8ce32c14]{display:flex;align-items:flex-start;justify-content:space-between;gap:16px;margin-bottom:20px;flex-wrap:wrap}.aksrt-page-header__main[data-v-8ce32c14]{flex:1 1 auto;min-width:0}.aksrt-page-header__title[data-v-8ce32c14]{margin:0 0 6px;color:#1a1a2e;font-size:22px;font-weight:700;letter-spacing:-.02em;line-height:1.3}.aksrt-page-header__desc[data-v-8ce32c14]{margin:0;color:#7a7f8a;font-size:13px;line-height:1.65}.aksrt-page-header__actions[data-v-8ce32c14]{flex:0 0 auto;display:flex;align-items:center;gap:8px}@media(max-width:768px){.aksrt-page-header[data-v-8ce32c14]{margin-bottom:14px;gap:10px}.aksrt-page-header__title[data-v-8ce32c14]{font-size:18px}.aksrt-page-header__desc[data-v-8ce32c14]{font-size:12px}.aksrt-page-header__actions[data-v-8ce32c14]{width:100%}}