Fix initial feed load performance and relay timing

[barrydeen]

Summary

• Await relay connections before subscribing: subscribeFeed() was firing immediately after updateRelays(), but WebSocket connections are async — relay.send() silently drops messages when not connected. Now waits for at least 3 relays (5s timeout).
• Reduce per-relay limit 100→25: With outbox routing to 30+ scored relays, limit=100 per relay meant thousands of events flooding in simultaneously, each triggering processRelayEvent → addEvent → StateFlow → recompose. 25 per relay is plenty for the initial screen; loadMore() handles pagination.
• Tighten adaptive time windows: Previous windows were too aggressive for small follow counts (7 days for ≤20 follows). Now: 2d/12h/4h/1h/15m instead of 7d/48h/12h/3h/30m.
• Fix stale relay index in reconnectAll(): Dead ephemeral relays were removed from ephemeralRelays but not relayIndex, causing sends to dead relays on subsequent lookups.
• Fix O(n) → O(1): Replace !in ephemeralRelays.values scan with containsKey lookup.

Test plan

• Open app fresh — feed should load quickly with ~20-30 recent notes, no jank
• Scroll down to trigger loadMore — older notes should paginate in smoothly
• Switch to relay feed and back to follows — should show same notes
• Account with few follows (≤20) — should see recent posts, not week-old content
• Account with many follows (300+) — should load fast with tight 4h window
• Background and resume — feed should reload correctly