feat: message reactions + link/image previews

Reactions (full stack):
- Wire: MsgReaction peer message (reaction_mid, reaction_emoji)
- Store: reactions table (mid, emoji, from_peer), SaveReaction, ReactionsForRoom
- Daemon: CmdSendReaction IPC command; EvtReaction IPC event; stored reactions
  replayed to newly-connected IPC clients alongside history
- Web UI: reactions state (mid → emoji → [fromId]); hover a message to reveal
  a + button; click opens a 6-emoji picker (👍❤️😂😮😢🙏); reaction chips
  appear below the message with count and tooltip; own reactions highlighted

Link/image previews:
- URLs in message text rendered as clickable <a> links
- Image URLs (.jpg/.jpeg/.png/.gif/.webp/blob:/data:image) rendered as inline
  thumbnails (max 320×200px) below the link

Also scoped push notifications architecture in FUTURE.md.

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

internal/mesh/peer.go

@@ -307,6 +307,18 @@ func dispatchPeerMessage(msg proto.PeerMessage, from proto.PeerID, m *Mesh) {
 	case proto.MsgHistoryChunk:
 		go m.HandleHistoryChunk(msg.Room, msg.History)
 
+	case proto.MsgReaction:
+		if msg.ReactionMID == "" || msg.ReactionEmoji == "" {
+			return
+		}
+		m.SaveReaction(msg.ReactionMID, msg.ReactionEmoji, string(from))
+		m.emit(proto.IpcMessage{
+			Type:          proto.EvtReaction,
+			PeerID:        peerIDPtr(from),
+			ReactionMID:   msg.ReactionMID,
+			ReactionEmoji: msg.ReactionEmoji,
+		})
+
 	case proto.MsgPing:
 		log.Printf("mesh: ping from %s", from.Short())
 	case proto.MsgPong: