perf: improve SEO and accessibility for PageSpeed

- Add meta description tag with theme_settings.site_description fallback
- Add site_description field to ThemeSettings schema
- Fix color contrast on tertiary text (WCAG AA compliance)
  - Clean mood: #a3a3a3 → #737373
  - Warm mood: #a8a29e → #78716c
  - Cool mood: #94a3b8 → #64748b
- Add width/height attributes to product images to prevent CLS:
  - Product cards: 600x600
  - Cart items: 96x96
  - Gallery thumbnails: 150x150
  - Lightbox: 1200x1200

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

lib/simpleshop_theme/settings/theme_settings.ex

@@ -16,6 +16,7 @@ defmodule SimpleshopTheme.Settings.ThemeSettings do
 
     # Branding
     field :site_name, :string, default: "Store Name"
+    field :site_description, :string, default: "Discover unique products and original designs."
     field :logo_mode, :string, default: "text-only"
     field :logo_image_id, :binary_id
     field :logo_size, :integer, default: 36
@@ -62,6 +63,7 @@ defmodule SimpleshopTheme.Settings.ThemeSettings do
       :header_layout,
       :accent_color,
       :site_name,
+      :site_description,
       :logo_mode,
       :logo_image_id,
       :logo_size,