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/theme/css_generator.ex

@@ -58,7 +58,7 @@ defmodule SimpleshopTheme.Theme.CSSGenerator do
       --t-surface-overlay: rgba(255, 255, 255, 0.95);
       --t-text-primary: #171717;
       --t-text-secondary: #525252;
-      --t-text-tertiary: #a3a3a3;
+      --t-text-tertiary: #737373;
       --t-text-inverse: #ffffff;
       --t-border-default: #e5e5e5;
       --t-border-subtle: #f0f0f0;
@@ -73,7 +73,7 @@ defmodule SimpleshopTheme.Theme.CSSGenerator do
       --t-surface-overlay: rgba(253, 248, 243, 0.95);
       --t-text-primary: #1c1917;
       --t-text-secondary: #57534e;
-      --t-text-tertiary: #a8a29e;
+      --t-text-tertiary: #78716c;
       --t-text-inverse: #ffffff;
       --t-border-default: #e7e0d8;
       --t-border-subtle: #f0ebe4;
@@ -88,7 +88,7 @@ defmodule SimpleshopTheme.Theme.CSSGenerator do
       --t-surface-overlay: rgba(244, 247, 251, 0.95);
       --t-text-primary: #0f172a;
       --t-text-secondary: #475569;
-      --t-text-tertiary: #94a3b8;
+      --t-text-tertiary: #64748b;
       --t-text-inverse: #ffffff;
       --t-border-default: #d4dce8;
       --t-border-subtle: #e8eff5;