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>

EXTENSIONS.md

@@ -304,3 +304,74 @@ without a `mid` are assigned one at receive time and are not gossipped.
 
 Emitted once per room after a `history_chunk` is fully processed. The UI
 should render these messages with a visual separator from live messages.
+
+---
+
+## EXT-008 — Message Reactions
+
+### Wire message (`PeerMessage`)
+
+```json
+{
+  "type": "reaction",
+  "reaction_mid": "<32-hex mid of the target message>",
+  "reaction_emoji": "👍"
+}
+```
+
+Sent on the normal mesh DataChannel (same as `chat`). No signing beyond
+the existing channel-level encryption.
+
+### Semantics
+
+- A reaction is idempotent: the same `(mid, emoji, from_peer)` triple is
+  stored with `INSERT OR IGNORE` — receiving a duplicate is a no-op.
+- There is no "un-react" wire message. Toggling off a reaction in the UI
+  is a local-only operation in the current implementation.
+- `reaction_mid` must reference a message that exists in the local store;
+  unknown mids are silently ignored.
+
+### Storage
+
+SQLite table added as a migration:
+
+```sql
+CREATE TABLE IF NOT EXISTS reactions (
+  mid        TEXT NOT NULL,
+  emoji      TEXT NOT NULL,
+  from_peer  TEXT NOT NULL,
+  reacted_at DATETIME NOT NULL,
+  PRIMARY KEY (mid, emoji, from_peer)
+)
+```
+
+### IPC
+
+**Command** — send a reaction (daemon and browser mode):
+```json
+{ "type": "send_reaction", "network_id": "...", "reaction_mid": "<hex>", "reaction_emoji": "👍" }
+```
+
+**Event** — reaction received or replayed from history:
+```json
+{ "type": "reaction", "network_id": "...", "peer_id": "<64-hex>", "reaction_mid": "<hex>", "reaction_emoji": "👍" }
+```
+
+Stored reactions are replayed as `reaction` IPC events when history is
+loaded (`sendStoredHistory`), so the UI always sees reactions alongside
+their messages.
+
+### History replay
+
+When `sendStoredHistory` sends a `history_chunk`, it also queries
+`ReactionsForRoom` and emits one `reaction` event per stored reaction so
+clients receive the full reaction state on reconnect.
+
+### Browser mode
+
+`browser.ts` mirrors the daemon behaviour independently:
+
+- `PeerConn.sendReaction(mid, emoji)` broadcasts `{ type: "reaction", reaction_mid, reaction_emoji }` over the DataChannel.
+- Incoming `reaction` wire frames are dispatched as `reaction` IPC events.
+- `BrowserAdapter.send()` handles `send_reaction` commands and both broadcasts to all peers and emits a local `reaction` event.
+- `sendChat` includes the `mid` in the wire frame so reactions can reference it correctly across peers.