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>

web/src/types.ts

@@ -86,6 +86,8 @@ export type IpcMsgType =
   | 'room_created'
   | 'create_room'
   | 'resumable_transfers'
+  | 'send_reaction'
+  | 'reaction'
 
 export interface IpcMessage {
   type: IpcMsgType
@@ -133,4 +135,7 @@ export interface IpcMessage {
   messages?: ChatMessage[]
   // resumable_transfers
   resumable_files?: Array<{ name: string; sha256: string; from: string; size: number; offset: number }>
+  // reaction
+  reaction_mid?: string
+  reaction_emoji?: string
 }