feat: chat — ⋮ room menu, typing indicator, online presence, read receipts

- RoomRow: ⋮ button (hover) for pin/unpin context menu
- MessageBubble: right-click context menu with emoji reactions + edit/delete
- Typing indicator: debounced setTyping, animated TypingDots, polling getTyping every 2s
- Online presence: heartbeat setPresence every 30s, green dot on avatars/header
- Read receipts: ✓/✓✓ for own messages in direct rooms via otherUserLastRead
- Migration 075: chat_presence + chat_typing tables
- Context menus clamped to viewport bounds

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

backend/migrations/075_chat_presence.sql

@@ -0,0 +1,17 @@
+-- Online presence
+CREATE TABLE IF NOT EXISTS chat_presence (
+  user_id   UUID        NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  hotel_id  UUID        NOT NULL REFERENCES hotels(id) ON DELETE CASCADE,
+  last_seen TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  PRIMARY KEY (user_id, hotel_id)
+);
+CREATE INDEX IF NOT EXISTS idx_chat_presence_hotel ON chat_presence(hotel_id, last_seen);
+
+-- Typing indicator (expires after 5s — queried with WHERE typing_at > NOW() - interval '5s')
+CREATE TABLE IF NOT EXISTS chat_typing (
+  user_id   UUID        NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  room_id   UUID        NOT NULL REFERENCES chat_rooms(id) ON DELETE CASCADE,
+  name      TEXT        NOT NULL,
+  typing_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  PRIMARY KEY (user_id, room_id)
+);