feat: rebrand accent color to Violet #6c3aed

Replace Amber brand color with Violet (#6c3aed) across the entire app.

- tokens.css: accent palette → Violet (light #6c3aed, dark #a78bfa)
- Logo, favicon, PWA icons, Apple touch icon regenerated
- GitHub Pages (index.html, install.html): accent + inline SVG updated
- generate-icons.js: gradient updated for future icon generation
- Semantic colors (warning, notes, meals) intentionally unchanged

docs/logo.svg

@@ -1,8 +1,8 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 160 160" fill="none">
   <defs>
     <linearGradient id="bg" x1="0" y1="0" x2="160" y2="160" gradientUnits="userSpaceOnUse">
-      <stop offset="0%" stop-color="#B45309"/>
-      <stop offset="100%" stop-color="#92400E"/>
+      <stop offset="0%" stop-color="#8b5cf6"/>
+      <stop offset="100%" stop-color="#6c3aed"/>
     </linearGradient>
   </defs>
   <rect width="160" height="160" rx="36" fill="url(#bg)"/>