Fix getPublicKey: Java returns hex not npub despite field name

NostrSigner.npubToHex() is called before putting result in "npub" field,
so the value is already a 64-char hex pubkey. Skip nip19.decode() and
use it directly. This was causing the "data must be at least 8 chars" error.

Co-authored-by: shakespeare.diy <assistant@shakespeare.diy>

src/lib/capacitorAmberSigner.ts

@@ -44,6 +44,13 @@ export async function getAmberPublicKeyViaPlugin(): Promise<string> {
 
   const { npub } = await plugin.getPublicKey(AMBER_PACKAGE);
 
+  // Despite the field being called "npub", the Java bridge calls npubToHex()
+  // before returning — so this is actually a hex pubkey already.
+  if (/^[0-9a-f]{64}$/i.test(npub)) {
+    return npub.toLowerCase();
+  }
+
+  // Fallback: if it somehow comes back as a real npub, decode it
   const decoded = nip19.decode(npub);
   if (decoded.type !== 'npub') throw new Error('Unexpected key format from Amber');
   return decoded.data as string;