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>

README.md

@@ -392,9 +392,16 @@ go run ./cmd/tui -network friends
 | `-join` | — | `waste:` invite string |
 | `-ipc` | `17337` | Daemon IPC port |
 
-**Key bindings:** `Tab`/`Shift+Tab` — switch rooms · `PgUp`/`PgDn` — scroll · `Enter` — send · `Ctrl+I` — generate invite · `Esc` — close overlay · `Ctrl+C` — quit
+**Key bindings:** `Tab`/`Shift+Tab` — switch rooms · `Ctrl+N` — cycle networks · `PgUp`/`PgDn` — scroll · `Enter` — send · `Ctrl+I` — generate invite · `Esc` — close overlay · `Ctrl+C` — quit
 
-**Slash commands:** `/room <name>` — create a new room (persisted in SQLite, restored on reconnect). Rooms with unread messages show a `*` prefix in the sidebar.
+**Slash commands:**
+- `/room <name>` — create a new room (persisted in SQLite, restored on reconnect)
+- `/join <name>` — join a new network at runtime (the `-network` flag is optional; start idle and `/join` to connect)
+- `/net <n|name>` — switch active network by index or name
+- `/react <emoji>` — react to the last message (use actual emoji characters, e.g. `/react 👍`)
+- `/react <n> <emoji>` — react to message `[n]` (line numbers shown next to each message)
+
+Rooms with unread messages show a `*` prefix in the sidebar.
 
 ---
 
@@ -419,6 +426,7 @@ Newline-delimited JSON on TCP port 17337 (or WebSocket on 17338).
 {"type":"create_room","room":"dev"}
 {"type":"set_download_dir","path":"/home/alice/Downloads"}               // change download base for current network
 {"type":"set_download_dir","network_id":"<16-hex>","path":"/home/alice/Downloads/friends"}
+{"type":"send_reaction","network_id":"...","reaction_mid":"<32-hex>","reaction_emoji":"👍"}
 {"type":"export_identity","passphrase":"..."}
 {"type":"import_identity","passphrase":"...","backup":"..."}
 ```
@@ -435,6 +443,7 @@ Newline-delimited JSON on TCP port 17337 (or WebSocket on 17338).
 {"type":"incoming_file","peer_id":"<64-hex>","offer":{"xid":"...","name":"notes.txt","size":1024,"sha256":"..."}}
 {"type":"file_complete","transfer_id":"...","path":"/downloads/notes.txt"}
 {"type":"room_created","network_id":"...","room":"dev"}
+{"type":"reaction","network_id":"...","peer_id":"<64-hex>","reaction_mid":"<32-hex>","reaction_emoji":"👍"}
 {"type":"identity_exported","backup":"..."}
 {"type":"error","error_message":"..."}
 ```