fix(nostr): accept an uppercase npub, reject a mixed-case one

BIP-173 defines a bech32 string as all lowercase or all uppercase, and
declares mixed case invalid. The decoder only looked up lowercase
characters in the charset, so an all-uppercase npub - the form QR
encoders emit, because uppercase fits the denser alphanumeric mode -
failed with "Invalid character" before the checksum was ever checked.

Normalise case once, after rejecting a mixed-case string outright.
ContactIdentityResolver already worked around this by lowercasing at the
call site; NostrClient.sendPrivateMessage does not, so an npub that came
in uppercase could not be messaged.

app/src/main/java/com/bitchat/android/nostr/Bech32.kt

@@ -22,14 +22,23 @@ object Bech32 {
     /**
      * Decode bech32 string
      * Returns (hrp, data) pair
+     *
+     * Per BIP-173 a bech32 string is either all lowercase or all uppercase;
+     * the uppercase form is what QR encoders emit, since it fits the compact
+     * alphanumeric mode. Mixed case is invalid and is rejected.
      */
     fun decode(bech32String: String): Pair<String, ByteArray> {
-        val separatorIndex = bech32String.lastIndexOf('1')
+        val hasLower = bech32String.any { it.isLowerCase() }
+        val hasUpper = bech32String.any { it.isUpperCase() }
+        require(!(hasLower && hasUpper)) { "Mixed case" }
+        val normalized = bech32String.lowercase()
+
+        val separatorIndex = normalized.lastIndexOf('1')
         require(separatorIndex >= 0) { "No separator found" }
-        
-        val hrp = bech32String.substring(0, separatorIndex)
-        val dataString = bech32String.substring(separatorIndex + 1)
-        
+
+        val hrp = normalized.substring(0, separatorIndex)
+        val dataString = normalized.substring(separatorIndex + 1)
+
         // Validate HRP contains only ASCII
         require(hrp.all { it.code < 128 }) { "Invalid HRP characters" }