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>

priv/repo/migrations/20260224162707_create_favicon_variants.exs

@@ -0,0 +1,18 @@
+defmodule Berrypod.Repo.Migrations.CreateFaviconVariants do
+  use Ecto.Migration
+
+  def change do
+    create table(:favicon_variants, primary_key: false) do
+      add :id, :binary_id, primary_key: true
+      add :source_image_id, references(:images, type: :binary_id, on_delete: :nilify_all)
+      add :png_32, :binary
+      add :png_180, :binary
+      add :png_192, :binary
+      add :png_512, :binary
+      add :svg, :text
+      add :generated_at, :utc_datetime
+
+      timestamps(type: :utc_datetime)
+    end
+  end
+end