Audit LOW: empty offer, attachment id, QR binding, skipped-key order

Same empty-offer and attachment-id rules as iOS. The QR scanner now
binds a scanned code by its fnid rather than the handle even on the home
relay — the fnid certifies itself, a handle is a name the relay maps,
so resolving by handle handed the relay the choice of who you scanned.
And a skipped message key is consumed only after its tag verifies: it
was defused today by the caller discarding the session, but any future
in-memory ratchet cache would have turned it into real message loss.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/kotlin/io/flamenet/messenger/app/MainActivity.kt

@@ -724,12 +724,13 @@ internal fun ColumnScope.SignedIn(
             onFound = { raw ->
                 scanning = false
                 parseScannedAddress(raw)?.let { card ->
-                    // A same-relay code that carries the handle can go through
-                    // the ordinary username lookup; anything else fills the
-                    // field with the address it named.
-                    scannedPrefill = if (card.handle != null &&
-                        card.relay.equals(phase.relay, ignoreCase = true)
-                    ) card.handle else "${card.fnid}@${card.relay}"
+                    // 🔴 **Bind by the fnid, never the handle (audit LOW).**
+                    // The fnid is derived from the account key and certifies
+                    // itself; a handle is a name the RELAY maps to an account,
+                    // so resolving a scanned code by handle handed the relay
+                    // the choice of who you just scanned. iOS always binds the
+                    // address; this now matches it, same relay or not.
+                    scannedPrefill = "${'$'}{card.fnid}@${'$'}{card.relay}"
                     addingContact = true
                 }
             },