feat: WebSocket heartbeat, multi-vault subscription, and connection s…

[Iceeyyou2]

What was done per issue

closes #252 — Heartbeat/ping-pong

• Both VaultEventSocket implementations now reply to server ping frames with a pong text frame
• Client sends a heartbeat ping every 30 s; a failed send immediately triggers the reconnect-with-backoff path
  instead of waiting for the OS to surface the dead connection
• Tests: test_pingMessage_sendsPongReply, test_silentConnectionDeath_detectedViaHeartbeatFailure on both
  platforms

closes #253 — Multi-vault subscription

• New subscribe / subscribed message types in api-contract.md
• iOS: new connect(vaultIDs:) overload sends a subscribe message post-connect for IDs beyond the first
• Android: VaultEventSocket.events(List) overload; VaultViewModel.subscribeToEvents() uses the
  multiplexed path automatically
• Tests: subscribe message content, event routing to correct vault when multiplexed

closes #254 — Backoff jitter

• Already fully implemented on both platforms — no code changes needed
• Documented the formula in shared/api-contract.md §Backoff/Jitter Formula

closes #255 — Connection status indicator

• iOS: VaultStore.socketConnectionState (@published), wired via onStateChange; ConnectionStatusBadge added to
  VaultDetailView's Overview section
• Android: VaultEventSocket.ConnectionState enum + StateFlow; VaultUiState.socketConnectionState;
  ConnectionStatusBadge chip in VaultDetailScreen
• Tests: all four states verified through VaultStore/VaultViewModel

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