add favicon and site icon generation from uploaded images

Upload a source image (PNG, JPEG, or SVG) and get a complete favicon
setup: PNG variants at 32, 180, 192, 512px served from DB via
FaviconController with ETag caching, SVG favicon for vector sources,
dynamic site.webmanifest, and theme-color meta tag. Theme editor gains
a site icon section with "use logo as icon" toggle, dedicated icon
upload, short name, and background colour picker.

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

lib/berrypod/settings/theme_settings.ex

@@ -41,6 +41,12 @@ defmodule Berrypod.Settings.ThemeSettings do
     field :product_text_align, :string, default: "left"
     field :image_aspect_ratio, :string, default: "square"
 
+    # Favicon / site icon
+    field :use_logo_as_icon, :boolean, default: true
+    field :icon_image_id, :binary_id
+    field :favicon_short_name, :string, default: ""
+    field :icon_background_color, :string, default: "#ffffff"
+
     # Feature toggles
     field :announcement_bar, :boolean, default: true
     field :sticky_header, :boolean, default: false
@@ -83,6 +89,10 @@ defmodule Berrypod.Settings.ThemeSettings do
       :card_shadow,
       :product_text_align,
       :image_aspect_ratio,
+      :use_logo_as_icon,
+      :icon_image_id,
+      :favicon_short_name,
+      :icon_background_color,
       :announcement_bar,
       :sticky_header,
       :hover_image,