mobile nav, locale-preserving navigation, accent button contrast

src/components/ui/button.tsx

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+import { forwardRef } from "react";
+
+/**
+ * Shared button primitive.
+ *
+ * Why this exists
+ * ---------------
+ * The accent fill (#00d4aa) is bright; white text on it measures ~1.9:1,
+ * which fails WCAG even for large/UI text. Dark text (surface-0) on the
+ * same accent is ~10:1. The codebase had ~40 hand-rolled accent buttons,
+ * most using `text-white`. This component centralises the correct token
+ * (`text-surface-0` on accent) so the contrast can't drift again — reach
+ * for `<Button>` instead of re-deriving the class string.
+ */
+
+type Variant = "primary" | "secondary" | "ghost" | "danger";
+type Size = "sm" | "md";
+
+const BASE =
+  "inline-flex items-center justify-center gap-1.5 font-medium rounded-lg " +
+  "transition-colors cursor-pointer focus:outline-none focus-visible:ring-2 " +
+  "focus-visible:ring-accent/50 disabled:opacity-50 disabled:cursor-not-allowed";
+
+const VARIANTS: Record<Variant, string> = {
+  // surface-0 (dark) text — the contrast-correct pairing for the accent.
+  primary: "bg-accent text-surface-0 hover:bg-accent-dim shadow-sm shadow-accent/20",
+  secondary:
+    "bg-surface-2 text-text-primary border border-border hover:bg-surface-3 hover:border-border-active",
+  ghost: "text-text-secondary hover:text-text-primary hover:bg-surface-2",
+  danger: "bg-error text-surface-0 hover:opacity-90",
+};
+
+const SIZES: Record<Size, string> = {
+  sm: "text-xs px-3 py-1.5",
+  md: "text-sm px-4 py-2",
+};
+
+export interface ButtonProps
+  extends React.ButtonHTMLAttributes<HTMLButtonElement> {
+  variant?: Variant;
+  size?: Size;
+}
+
+export const Button = forwardRef<HTMLButtonElement, ButtonProps>(
+  function Button(
+    { variant = "primary", size = "md", className = "", type = "button", ...rest },
+    ref
+  ) {
+    return (
+      <button
+        ref={ref}
+        type={type}
+        className={`${BASE} ${VARIANTS[variant]} ${SIZES[size]} ${className}`}
+        {...rest}
+      />
+    );
+  }
+);