switch mockup generator to UK print providers and add --replace flag

- Prefer Print Clever (72) for canvas, Monster Digital (29) for
  apparel and mugs where available, fall back to default providers
- Rename Art Print products to Canvas (new Satin Canvas blueprint)
- Add Canvas Prints category in Printify tag extraction
- Add --replace/-r flag to purge existing Printify products before
  generating (with interactive confirmation)
- Add purge_all_products/1 to generator module
- Remove old art print mockups, add new canvas + extra provider mockups

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

lib/simpleshop_theme/providers/printify.ex

@@ -418,10 +418,11 @@ defmodule SimpleshopTheme.Providers.Printify do
 
     cond do
       has_tag?(tags, ~w[t-shirt tshirt shirt hoodie sweatshirt apparel clothing]) -> "Apparel"
+      has_tag?(tags, ~w[canvas]) -> "Canvas Prints"
       has_tag?(tags, ~w[mug cup blanket pillow cushion homeware homewares home]) -> "Homewares"
       has_tag?(tags, ~w[notebook journal stationery]) -> "Stationery"
       has_tag?(tags, ~w[phone case bag tote accessories]) -> "Accessories"
-      has_tag?(tags, ~w[art print poster canvas wall]) -> "Art Prints"
+      has_tag?(tags, ~w[art print poster wall]) -> "Art Prints"
       true -> List.first(raw["tags"])
     end
   end