fix(auth)!: change timestamp to ms since Unix epoch for effective replay protection

auth.proto

@@ -19,19 +19,24 @@ message Auth {
   // persists it across launches.
   bytes client_id = 1;
 
-  // Minutes since the project epoch (2024-01-01 UTC), used as a
-  // replay-prevention nonce. The Node accepts the message only if
-  // this value is within one minute of its own clock.
-  uint32 timestamp = 2;
+  // Milliseconds since the Unix epoch, used as a replay-prevention
+  // nonce. The Node accepts the message only if this value is
+  // within a small clock-skew window (typically 1–2 seconds) of its
+  // own clock. Millisecond granularity ensures a unique nonce per
+  // Auth message; unlike the previous minute-based timestamp, an
+  // attacker cannot replay a captured frame within a multi-minute
+  // window.
+  uint64 timestamp = 2;
 
   // Truncated HMAC-SHA256 (12 bytes) over (client_id || timestamp),
-  // computed with the pre-shared key issued to this client. Proves
-  // possession of the PSK without revealing it. Receivers MUST
-  // assert len(hmac) == 12 and reject the Auth before performing any
-  // comparison; a shorter or absent value weakens authentication
-  // non-deterministically across implementations. Implementations
-  // MUST also verify client_id is exactly 9 bytes before computing
-  // or checking this tag.
+  // computed with the pre-shared key issued to this client. The
+  // timestamp is serialised as 8 bytes in big-endian (network byte
+  // order) before concatenation. Proves possession of the PSK
+  // without revealing it. Receivers MUST assert len(hmac) == 12 and
+  // reject the Auth before performing any comparison; a shorter or
+  // absent value weakens authentication non-deterministically across
+  // implementations. Implementations MUST also verify client_id is
+  // exactly 9 bytes before computing or checking this tag.
   bytes hmac = 3;
 
   // Free-form device label reported by the client (e.g. "SM-G998B",