feat: add UI primitive library — Button, Card, Input, Badge, Modal, Toast

- Add cn() utility for class merging in lib/utils.ts
- Button: primary/secondary/ghost/danger variants, sm/md/lg sizes, disabled/loading states
- Card: gradient-card background with optional header/footer slots
- Input: text/email/password/number types with label, error, helper text, focus ring
- Badge: default/success/warning/error/info variants
- Modal: Portal-based dialog with focus trap, ESC/backdrop close, animations
- Toast: ToastProvider context with show/dismiss/auto-dismiss and variant support
- Barrel export via index.ts
- 46 unit tests across all primitives
- Configure vitest with vite-plugin-solid for JSX support

web/src/components/ui/index.ts

@@ -0,0 +1,6 @@
+export { default as Button } from "./Button";
+export { default as Card } from "./Card";
+export { default as Input } from "./Input";
+export { default as Badge } from "./Badge";
+export { default as Modal } from "./Modal";
+export { ToastProvider, useToast } from "./Toast";