feat: show broadcast progress when publishing notes

[barrydeen]

Summary

• Instead of fire-and-forget publishing, shows a floating toast with live relay confirmation count ("Broadcasting 0/5" → "Published to 5 relays")
• If no relays are connected, shows an error on the compose screen instead of silently navigating back
• Toast appears globally (all screens) so users can continue browsing while broadcast completes

Test plan

• Publish a note normally — toast should appear at bottom showing relay confirmations, then fade
• Reply to a note — same broadcast toast on the thread screen
• Enable airplane mode, try to publish — should see error "No relays connected" and stay on compose screen
• Verify toast styling matches bottom nav color and is compact