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/proto/proto.go

@@ -51,6 +51,7 @@ const (
 	MsgPong            MsgType = "pong"
 	MsgHistoryRequest  MsgType = "history_request"
 	MsgHistoryChunk    MsgType = "history_chunk"
+	MsgReaction        MsgType = "reaction"
 )
 
 // PmMessage is a private message sent directly over a single peer link (§8 "pm").
@@ -97,6 +98,10 @@ type PeerMessage struct {
 	// history_chunk fields
 	History     []HistoryEntry `json:"history,omitempty"`
 	HistoryDone bool           `json:"history_done,omitempty"`
+
+	// reaction fields
+	ReactionMID   string `json:"reaction_mid,omitempty"`
+	ReactionEmoji string `json:"reaction_emoji,omitempty"`
 }
 
 // ResumableFile describes a partially-downloaded file found on daemon startup.
@@ -280,6 +285,7 @@ const (
 	CmdListShares     IpcMsgType = "list_shares"       // returns shares_list event
 	CmdCreateRoom       IpcMsgType = "create_room"        // field: room (name)
 	CmdSetDownloadDir   IpcMsgType = "set_download_dir"   // set per-network download directory at runtime; fields: network_id, path
+	CmdSendReaction     IpcMsgType = "send_reaction"      // fields: network_id, reaction_mid, reaction_emoji
 
 	// Events (daemon → UI)
 	EvtMessageReceived  IpcMsgType = "message_received"
@@ -302,6 +308,7 @@ const (
 	EvtRoomCreated      IpcMsgType = "room_created"    // field: room (name)
 	EvtHistoryLoaded      IpcMsgType = "history_loaded"       // fields: room, messages
 	EvtResumableTransfers IpcMsgType = "resumable_transfers"  // field: resumable_files
+	EvtReaction           IpcMsgType = "reaction"             // fields: reaction_mid, reaction_emoji, peer_id
 )
 
 // NetworkInfo summarises one joined network for state_snapshot and network_joined events.
@@ -359,6 +366,8 @@ type IpcMessage struct {
 	Files          []FileEntry    `json:"files,omitempty"`
 	Messages        []ChatMessage   `json:"messages,omitempty"`          // history_loaded
 	ResumableFiles  []ResumableFile `json:"resumable_files,omitempty"`   // resumable_transfers
+	ReactionMID     string          `json:"reaction_mid,omitempty"`      // reaction
+	ReactionEmoji   string          `json:"reaction_emoji,omitempty"`    // reaction
 	Shares         []ShareEntry `json:"shares,omitempty"`
 	ShareNetworks  []string     `json:"network_ids,omitempty"` // for add_share command: scope to specific network IDs, or ["*"] for global
 	// export_identity / import_identity