add header background contrast warning and improve branding UX

- extract dominant colors from header images during optimization
- calculate WCAG contrast ratios against theme text color
- show warning in theme editor when text may be hard to read
- prevent hiding shop name when no logo is uploaded
- auto-enable shop name when logo is deleted
- fix image cache invalidation on delete
- add missing .hidden utility class

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

priv/repo/migrations/20260308214935_add_dominant_colors_to_images.exs

@@ -0,0 +1,9 @@
+defmodule Berrypod.Repo.Migrations.AddDominantColorsToImages do
+  use Ecto.Migration
+
+  def change do
+    alter table(:images) do
+      add :dominant_colors, :string
+    end
+  end
+end