feat(admin): implement product management — list, create, edit, archive (Plan 03)

Covers checklist items 3.1–3.4, 3.10–3.11 (product list, create, edit,
archive/restore, SEO fields, admin search).

Backend (convex/products.ts):
- Extended create/update with shortDescription, brand, attributes,
  seoTitle, seoDescription, canonicalSlug
- Both mutations now set createdAt/updatedAt timestamps
- Added getByIdForAdmin (admin-only, returns full product with relations)

UI — new pages:
- products/page.tsx: table with debounced search, column visibility
  dropdown, client-side sort, 10-row skeleton, load-more pagination,
  row preview dialog, per-row actions menu
- products/new/page.tsx: create product page
- products/[id]/edit/page.tsx: pre-populated edit page with archive button

UI — new components:
- ProductForm: shared form (create + edit); zod + react-hook-form,
  auto-slug, collapsible Attributes + SEO sections, submit spinner
- ProductPreviewDialog: read-only full-product dialog
- ProductActionsMenu: kebab menu (Edit link + Archive AlertDialog)

ShadCN components installed: table, badge, alert-dialog, dialog,
scroll-area, form, select, label, checkbox, textarea

Also:
- Updated CLAUDE.md: form submit buttons must use inline SVG spinner
  with data-icon="inline-start"; link-styled buttons use buttonVariants
  on <Link> (Button render prop not in TS types)
- Updated docs: checklist and plan marked complete

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

package-lock.json

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