Trial banner polish and save-bar improvements

• ✨ New: Loading skeletons throughout the admin for a smoother feel between pages.
• ✨ New: Confirmation prompt when leaving a page with unsaved changes — no more accidentally losing work.
• 🚀 Improved: Trial banner is more compact and is hidden on the plan page itself (where it would be redundant).
• 🐛 Fixed: Pro trial activation no longer hits an edge-case error when switching from trial to paid.
• 🚀 Improved: Product Limits page — tab and limit sections rebuilt with native Shopify web components for a faster, more consistent look.