feat: add reusable InfoTip component for field-level help

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

frontend/src/components/common/InfoTip.tsx

@@ -0,0 +1,14 @@
+interface InfoTipProps {
+  text: string
+}
+
+export function InfoTip({ text }: InfoTipProps) {
+  return (
+    <span
+      className="inline-flex items-center justify-center h-3.5 w-3.5 rounded-full border border-muted-foreground/40 text-[9px] text-muted-foreground cursor-help shrink-0"
+      title={text}
+    >
+      i
+    </span>
+  )
+}