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.

PRODUCT.md

@@ -26,7 +26,7 @@ Most radio apps play streams but discard what's on them. SatsAmp treats the live
 ## Operating Context
 
 - Single-user personal device use; headphone and speaker listening.
-- Streams are live and can drop; playback is a foreground service with notification controls.
+- Streams are live and can drop; playback is a foreground service with notification controls. Swiping the task away stops playback and clears the notification (closed means stopped); screen-off listening with the task alive keeps playing.
 - Library is local (Room DB, `radamp.db`); saved songs, playlists, and imported files live on-device.
 - Stations are built-in defaults plus user-added streams (direct URL or M3U playlist import).
 
@@ -45,7 +45,7 @@ Most radio apps play streams but discard what's on them. SatsAmp treats the live
 
 ## Evidence on Hand
 
-- 43 built-in stations, all verified live 2026-09-07 (HTTP 200 + audio payload; probe results were ephemeral in /tmp, not committed).
+- 11 built-in stations (cliamp.stream music channels in playlist order: Lofi → … → NCS Chill), all verified live 2026-09-09 (HTTP 200 + audio payload; probe results were ephemeral, not committed). Media Monarchy ships as a built-in stream in the Streams tab instead (talk, not music).
 - Working recorder pipeline (`recording/`: IcyStreamReader, SongBoundaryDetector, RollingAudioBuffer, MetadataEmbedder).
 - Design scratch work in `design/` (glass theme CSS + demo).
 - Absences that must not be fabricated: no testimonials, no user base, no press, no pricing, no privacy policy text.