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.css

@@ -194,7 +194,7 @@
   --card-foreground: oklch(0.95 0.01 280);
   --border: oklch(0.2 0.05 280);
   --input: oklch(1 0 0 / 0.15);
-  --primary: oklch(0.65 0.25 320);
+  --primary: oklch(65.054% 0.25033 319.934);
   --primary-foreground: oklch(0.98 0.01 320);
   --secondary: oklch(0.15 0.05 260);
   --secondary-foreground: oklch(0.9 0.02 260);