feat(nostr): Amber external-signer login, pubkey import, ephemeral zap fallback

NIP-55 get_public_key flow (nostrsigner: intent) with signer package
stored for future requests; manual npub import fallback; sign-out.
Rename user-facing Decentralized labels to Nostr. Fix Wavlake track
zaps for external-signer logins via canSignLocally + ephemeral keys.
Artist address resolution + zap buttons in Discover search.

app/proguard-rules.pro

@@ -1,2 +1,13 @@
 -dontwarn javax.annotation.**
 -keep class com.rada.data.local.entity.** { *; }
+
+# jaudiotagger maps tag frames via reflection; without keeps the release
+# minifier breaks its writer (silent NPE, files saved untagged).
+-keep class org.jaudiotagger.** { *; }
+-keepattributes Signature, InnerClasses, EnclosingMethod
+# jaudiotagger also references desktop-only image/test APIs that Android
+# never executes (artwork scaling, test helpers).
+-dontwarn org.jaudiotagger.**
+-dontwarn java.awt.**
+-dontwarn javax.imageio.**
+-dontwarn javax.swing.**