Redesign frontend UI with premium SaaS styling and app icon

- Restyle design system: refined color palette, 16px base font, antialiased
  text rendering, improved typography hierarchy across all pages
- Update base components (button, input, card, checkbox, dialog, sidebar)
  with modern rounded corners, subtle shadows, and smooth transitions
- Redesign layout: remove header bar, move controls to sidebar footer,
  add two-column todo dashboard with stats and upcoming reminders
- Replace hardcoded slate colors with design token system throughout
- Add app icon (favicon, apple-icon, sidebar logo) from notify_icon.png
- Improve typography: page titles 20px, section titles 18px, sidebar
  nav 14px, stats 24px semibold, body text with proper line-height

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

frontend/tailwind.config.ts

@@ -56,9 +56,11 @@ const config: Config = {
         },
       },
       borderRadius: {
-        lg: "var(--radius)",
-        md: "calc(var(--radius) - 2px)",
-        sm: "calc(var(--radius) - 4px)",
+        "2xl": "calc(var(--radius) + 4px)",
+        xl: "var(--radius)",
+        lg: "calc(var(--radius) - 2px)",
+        md: "calc(var(--radius) - 4px)",
+        sm: "calc(var(--radius) - 6px)",
       },
     },
   },