feat: reactions, link previews, mobile layout, TUI multi-network + /react usage hints

- TUI /react shows usage hint on bad/missing args instead of silently no-oping
- README: document /join /net /react slash commands and Ctrl+N keybinding
- README: add send_reaction IPC command and reaction IPC event to protocol reference
- FUTURE.md: mark reactions, link previews, responsive mobile layout, TUI
  multi-network and TUI reactions as shipped; add prose sections for each
- EXTENSIONS.md: add EXT-008 documenting reaction wire protocol, SQLite
  schema, IPC commands/events, history replay, and browser-mode implementation

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

cmd/tui/main.go

@@ -636,6 +636,10 @@ func (m model) doSend(cmds []tea.Cmd) (model, []tea.Cmd) {
 				emoji = parts[1]
 			}
 		}
+		if emoji == "" {
+			m.errMsg = "usage: /react <emoji>  or  /react <n> <emoji>   (e.g. /react 👍  or  /react 3 ❤️)"
+			return m, cmds
+		}
 		msgs := m.messages[m.msgKey()]
 		var targetMid string
 		if targetIdx == -1 && len(msgs) > 0 {
@@ -643,14 +647,16 @@ func (m model) doSend(cmds []tea.Cmd) (model, []tea.Cmd) {
 		} else if targetIdx >= 0 && targetIdx < len(msgs) {
 			targetMid = msgs[targetIdx].mid
 		}
-		if targetMid != "" && emoji != "" {
-			cmds = append(cmds, sendIPC(m.enc, proto.IpcMessage{
-				Type:          proto.CmdSendReaction,
-				NetworkID:     m.activeNetworkID(),
-				ReactionMID:   targetMid,
-				ReactionEmoji: emoji,
-			}))
+		if targetMid == "" {
+			m.errMsg = "usage: /react <emoji>  or  /react <n> <emoji>   (e.g. /react 👍  or  /react 3 ❤️)"
+			return m, cmds
 		}
+		cmds = append(cmds, sendIPC(m.enc, proto.IpcMessage{
+			Type:          proto.CmdSendReaction,
+			NetworkID:     m.activeNetworkID(),
+			ReactionMID:   targetMid,
+			ReactionEmoji: emoji,
+		}))
 		return m, cmds
 	}