feat: add auto-select reply identity feature with settings and localization

lib/reply-identity.ts

@@ -0,0 +1,62 @@
+import type { Identity } from '@/lib/jmap/types';
+
+interface ReplyRecipient {
+  email?: string | null;
+}
+
+interface ReplyRecipients {
+  to?: ReplyRecipient[];
+  cc?: ReplyRecipient[];
+  bcc?: ReplyRecipient[];
+}
+
+function normalizeEmailAddress(email: string): string {
+  return email.trim().toLowerCase();
+}
+
+function normalizeBaseEmailAddress(email: string): string {
+  const normalized = normalizeEmailAddress(email);
+  const atIndex = normalized.indexOf('@');
+
+  if (atIndex <= 0) {
+    return normalized;
+  }
+
+  const localPart = normalized.slice(0, atIndex);
+  const domain = normalized.slice(atIndex + 1);
+  const plusIndex = localPart.indexOf('+');
+
+  return `${plusIndex >= 0 ? localPart.slice(0, plusIndex) : localPart}@${domain}`;
+}
+
+export function findReplyIdentityId(
+  identities: Identity[],
+  recipients?: ReplyRecipients,
+): string | null {
+  if (identities.length === 0 || !recipients) {
+    return null;
+  }
+
+  const receivedAddresses = [
+    ...(recipients.to || []),
+    ...(recipients.cc || []),
+    ...(recipients.bcc || []),
+  ]
+    .map((recipient) => recipient.email?.trim())
+    .filter((email): email is string => Boolean(email));
+
+  if (receivedAddresses.length === 0) {
+    return null;
+  }
+
+  const exactMatches = new Set(receivedAddresses.map(normalizeEmailAddress));
+  const exactIdentity = identities.find((identity) => exactMatches.has(normalizeEmailAddress(identity.email)));
+  if (exactIdentity) {
+    return exactIdentity.id;
+  }
+
+  const baseMatches = new Set(receivedAddresses.map(normalizeBaseEmailAddress));
+  const baseIdentity = identities.find((identity) => baseMatches.has(normalizeBaseEmailAddress(identity.email)));
+
+  return baseIdentity?.id ?? null;
+}