feat: add relay sets and favorite relays (NIP-51)

[barrydeen]

Summary

• Add NIP-51 support for named relay sets (kind 30002) and favorite relays (kind 10012)
• Relay picker shows favorites section, relay sets with combined multi-relay feed option, and individual relay selection
• Relay feed bar includes favorite star toggle and add-to-set pill for quick actions
• Relay detail screen has favorite toggle and add-to-set button with set picker dialog
• New RelaySetRepository with SharedPreferences persistence, event routing, and startup subscription

Test plan

• Open relay picker from feed type dropdown — verify favorites and relay sets sections appear
• Favorite a relay from the relay feed bar or detail screen — verify it appears in picker favorites section
• Create a relay set from relay picker or detail screen — verify it appears in relay sets section
• Add relays to a set — verify set expands to show individual relays
• Select "Combined Feed" on a relay set — verify events load from all relays in the set
• Select individual relay from a set — verify single relay feed works
• Verify kind 10012 and 30002 events are published to write relays