Add extended follows feed with network discovery

[barrydeen]

Summary

• Adds a new "Extended" feed type that expands the timeline to include 2nd-degree connections (friends-of-friends) filtered to those followed by at least 10 people in the user's network
• Full discovery flow with progress screen showing follow list fetching, network computation, relay list resolution, and greedy set-cover relay mapping
• Per-account caching with 24h staleness + follow list drift detection, so subsequent loads are instant
• Refresh button in the top bar when Extended feed is active to rebuild the network on demand

New files

• ExtendedNetworkRepository.kt — discovery algorithm, caching, outbox-model relay routing
• NetworkDiscoveryScreen.kt — progress UI during discovery

Modified files

• FeedViewModel.kt — EXTENDED_FOLLOWS feed type, event routing for extnet-* subs, resubscribe/loadMore branches
• FeedScreen.kt — "Extended" dropdown item, refresh button, empty state
• Navigation.kt — NETWORK_DISCOVERY route with auto-navigate on completion
• SubscriptionTracker.kt — extnet- priority prefix to bypass soft cap

Test plan

• Log in with an account following 50+ people
• Tap feed picker → "Extended" → verify discovery screen with progress
• After completion, verify feed loads posts from 2nd-degree follows
• Kill and restart → tap "Extended" → loads from cache instantly
• Tap refresh button → re-runs discovery and updates cache
• Switch accounts → verify extended network is per-account