feat(converter): add shareable conversion links via query parameters

lib/share-link.ts

@@ -0,0 +1,84 @@
+import { validateAmount } from "@/lib/validation";
+
+interface QueryParamReader {
+  get(name: string): string | null;
+}
+
+interface ConversionShareParams {
+  amount?: string;
+  fromCode?: string;
+  toCode?: string;
+}
+
+const CODE_PATTERN = /^[A-Z0-9]{2,12}$/;
+
+function normalizeCurrencyCode(value: string | null): string | undefined {
+  if (!value) {
+    return undefined;
+  }
+
+  const normalized = value.trim().toUpperCase();
+
+  if (!CODE_PATTERN.test(normalized)) {
+    return undefined;
+  }
+
+  return normalized;
+}
+
+function normalizeAmount(value: string | null): string | undefined {
+  if (!value) {
+    return undefined;
+  }
+
+  const trimmed = value.trim().replace(/\s+/g, "");
+
+  if (!trimmed || trimmed.length > 64) {
+    return undefined;
+  }
+
+  return validateAmount(trimmed).ok ? trimmed : undefined;
+}
+
+export function parseConversionShareParams(
+  searchParams: QueryParamReader,
+): ConversionShareParams {
+  const amount = normalizeAmount(searchParams.get("amount"));
+  const fromCode = normalizeCurrencyCode(searchParams.get("from"));
+  let toCode = normalizeCurrencyCode(searchParams.get("to"));
+
+  if (fromCode && toCode && fromCode === toCode) {
+    toCode = undefined;
+  }
+
+  return {
+    amount,
+    fromCode,
+    toCode,
+  };
+}
+
+export function buildConversionShareUrl(input: {
+  origin: string;
+  pathname: string;
+  amount: string;
+  fromCode: string;
+  toCode: string;
+}): string {
+  const url = new URL(input.pathname, input.origin);
+  const amount = normalizeAmount(input.amount) ?? "1";
+  const fromCode = normalizeCurrencyCode(input.fromCode);
+  const toCode = normalizeCurrencyCode(input.toCode);
+
+  url.searchParams.set("amount", amount);
+
+  if (fromCode) {
+    url.searchParams.set("from", fromCode);
+  }
+
+  if (toCode && toCode !== fromCode) {
+    url.searchParams.set("to", toCode);
+  }
+
+  return url.toString();
+}