Add SQLite message and peer persistence (internal/store)

Each daemon writes to <data-dir>/messages.db on startup. Messages received
or sent are stored immediately; duplicate mids (INSERT OR IGNORE) are safe
to call multiple times. Peer aliases are upserted on peer_connected and
again after hello verification when the real nick is known.

Schema
- messages(mid UNIQUE, room, from_peer, body, sent_at) — mid is the YAW/2
  dedup key added in the proto migration; index on (room, sent_at) for
  efficient per-room queries.
- peers(peer_id PK, alias, last_seen) — cache of every peer ever seen,
  used to resolve hex ids to names when peers are offline.

Wiring
- store.Open called in cmd/daemon/main.go, passed to mesh.New.
- mesh.Mesh holds *store.Store (nil-safe; persistence is optional).
- mesh.SaveMessage called in dispatchPeerMessage (incoming) and ipc
  CmdSendMessage (outgoing) so the local node's own messages are stored.
- mesh.UpdatePeerAlias called after hello verification updates the alias
  with the verified nick rather than the placeholder short-id.

Messages only accumulate from join time forward — no history replay to
late-joining peers; each node's view starts from when it connected.

test-network.sh: added SQLite verification block that queries each node's
DB after the test and prints message + peer counts.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/mesh/peer.go

@@ -128,6 +128,7 @@ func handleDCMessage(data []byte, from proto.PeerID, localID *crypto.Identity, m
 			conn.Info.PublicKey = hello.ID
 		}
 		m.mu.Unlock()
+		m.UpdatePeerAlias(from, hello.Nick)
 		m.Emit(proto.IpcMessage{
 			Type:   proto.EvtSessionReady,
 			PeerID: peerIDPtr(from),
@@ -148,6 +149,7 @@ func dispatchPeerMessage(msg proto.PeerMessage, from proto.PeerID, m *Mesh) {
 	switch msg.Type {
 	case proto.MsgChat:
 		if msg.Chat != nil {
+			m.SaveMessage(msg.Chat)
 			m.Emit(proto.IpcMessage{Type: proto.EvtMessageReceived, Message: msg.Chat})
 		}
 	case proto.MsgPeerGossip: