feat: add admin provider setup UI with improved product sync

- Add /admin/providers LiveView for connecting and managing POD providers
- Implement pagination for Printify API (handles all products, not just first page)
- Add parallel processing (5 concurrent) for faster product sync
- Add slug-based fallback matching when provider_product_id changes
- Add error recovery with try/rescue to prevent stuck sync status
- Add checksum-based change detection to skip unchanged products
- Add upsert tests covering race conditions and slug matching
- Add Printify provider tests
- Document Printify integration research (product identity, order risks,
  open source vs managed hosting implications)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/simpleshop_theme_web/components/core_components.ex

@@ -469,4 +469,68 @@ defmodule SimpleshopThemeWeb.CoreComponents do
   def translate_errors(errors, field) when is_list(errors) do
     for {^field, {msg, opts}} <- errors, do: translate_error({msg, opts})
   end
+
+  @doc """
+  Renders a modal dialog.
+
+  Uses daisyUI's modal component with proper accessibility.
+
+  ## Examples
+
+      <.modal id="confirm-modal">
+        Are you sure?
+        <:actions>
+          <button class="btn">Cancel</button>
+          <button class="btn btn-primary">Confirm</button>
+        </:actions>
+      </.modal>
+
+  """
+  attr :id, :string, required: true
+  attr :show, :boolean, default: false
+  attr :on_cancel, JS, default: %JS{}
+
+  slot :inner_block, required: true
+  slot :actions
+
+  def modal(assigns) do
+    ~H"""
+    <dialog
+      id={@id}
+      class="modal"
+      phx-mounted={@show && show_modal(@id)}
+      phx-remove={hide_modal(@id)}
+    >
+      <div class="modal-box max-w-lg">
+        <form method="dialog">
+          <button
+            class="btn btn-sm btn-circle btn-ghost absolute right-2 top-2"
+            phx-click={@on_cancel}
+            aria-label={gettext("close")}
+          >
+            <.icon name="hero-x-mark" class="size-5" />
+          </button>
+        </form>
+        {render_slot(@inner_block)}
+        <div :if={@actions != []} class="modal-action">
+          {render_slot(@actions)}
+        </div>
+      </div>
+      <form method="dialog" class="modal-backdrop">
+        <button phx-click={@on_cancel}>close</button>
+      </form>
+    </dialog>
+    """
+  end
+
+  def show_modal(js \\ %JS{}, id) when is_binary(id) do
+    js
+    |> JS.exec("showModal()", to: "##{id}")
+  end
+
+  def hide_modal(js \\ %JS{}, id) when is_binary(id) do
+    js
+    |> JS.exec("close()", to: "##{id}")
+    |> JS.pop_focus()
+  end
 end