Add end-to-end encryption for chat messages (X25519 / NaCl box)

Each device generates its own X25519 key pair on login (tweetnacl); the
private key never leaves localStorage or reaches our server. Messages are
encrypted client-side with the recipient's public key before being sent,
so the backend and anyone intercepting traffic only ever see ciphertext.

- src/lib/e2ee.ts: key generation/registration + encrypt/decrypt (NaCl box)
- Chat thread + matches list: encrypt on send, decrypt on receive/preview,
  with a graceful fallback when a message can't be decrypted on this device
  (e.g. after switching browsers) and full backward compat for legacy
  plaintext messages sent before this change
- Privacy settings page explains the guarantee and its one deliberate
  trade-off: there is no key backup/escrow, so clearing browser data or
  switching devices makes old messages permanently unreadable

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/lib/api.ts

@@ -51,6 +51,7 @@ export interface UserProfile {
   notify_matches: boolean;
   notify_messages: boolean;
   notify_likes: boolean;
+  public_key?: string;
 }
 
 export interface TokenPair {
@@ -168,7 +169,11 @@ export interface MatchView {
   photos: string[];
   created_at: string;
   last_message?: string;
+  last_message_cipher_text?: string;
+  last_message_nonce?: string;
   last_at?: string;
+  /** کلید عمومی X25519 طرف مقابل (base64)، برای رمزنگاری سرتاسری. اگه طرف مقابل هنوز کلید ثبت نکرده باشه خالیه. */
+  partner_public_key?: string;
 }
 
 export interface SwipeResult {
@@ -180,7 +185,10 @@ export interface MessageView {
   id: number;
   match_id: number;
   sender_id: number;
-  text: string;
+  /** فقط برای پیام‌های قدیمیِ قبل از رمزنگاری سرتاسری پر می‌شه */
+  text?: string;
+  cipher_text?: string;
+  nonce?: string;
   created_at: string;
   is_mine: boolean;
 }
@@ -494,9 +502,18 @@ export function listMessages(matchId: number) {
   return authRequest<MessageView[]>(`/matches/${matchId}/messages`);
 }
 
-export function sendMessage(matchId: number, text: string) {
+// cipherText/nonce قبلاً روی دستگاه فرستنده با کلید عمومی طرف مقابل
+// رمزنگاری شدن (src/lib/e2ee.ts)؛ متن خام پیام هیچ‌وقت به این تابع نمی‌رسه.
+export function sendMessage(matchId: number, cipherText: string, nonce: string) {
   return authRequest<MessageView>(`/matches/${matchId}/messages`, {
     method: "POST",
-    body: JSON.stringify({ text }),
+    body: JSON.stringify({ cipher_text: cipherText, nonce }),
+  });
+}
+
+export function updatePublicKey(publicKey: string) {
+  return authRequest<null>("/profile/public-key", {
+    method: "PUT",
+    body: JSON.stringify({ public_key: publicKey }),
   });
 }