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/src/app.html

@@ -9,7 +9,7 @@
     <link rel="preconnect" href="https://fonts.googleapis.com" />
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
     <link
-      href="https://fonts.googleapis.com/css2?family=Dancing+Script:wght@400..700&family=Noto+Sans:ital,wght@0,100..900;1,100..900&display=swap"
+      href="https://fonts.googleapis.com/css2?family=Noto+Sans:ital,wght@0,100..900;1,100..900&display=swap"
       rel="stylesheet"
     />