feat: rebrand to Sexy, restyle logo with gradient icon and updated assets

- Rename brand from SexyArt to Sexy throughout i18n locale
- Apply primary→accent gradient to SVG icon stroke
- Remove brand name text from logo, icon-only display
- Switch logo font to Noto Sans bold (default), drop Dancing Script
- Update favicons, app icons, webmanifest, and add logo.svg

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

packages/frontend/static/site.webmanifest

@@ -1,6 +1,6 @@
 {
-  "name": "Sexy.Art",
-  "short_name": "Sexy.Art",
+  "name": "Sexy",
+  "short_name": "Sexy",
   "icons": [
     {
       "src": "/android-chrome-192x192.png",
@@ -13,8 +13,8 @@
       "type": "image/png"
     }
   ],
-  "theme_color": "#ffffff",
-  "background_color": "#ffffff",
+  "theme_color": "#ce47eb",
+  "background_color": "#000000",
   "display": "standalone",
   "start_url": "/"
 }