feat(phase7): per-channel notification settings and read receipts

Phase 7.1 — Per-channel notification settings
- New notification_settings table (user_id, channel_id, level)
- GET/PUT/DELETE handlers under /channels/{channelID}/notifications
- Push dispatch and @mention loops filtered by notification level
- Frontend: bell icon per channel cycling all/mentions/none

Phase 7.4 — Read receipts
- New read_states table (user_id, channel_id, last_read_message_id)
- PUT /channels/{channelID}/read and GET /users/me/read-states
- Auto-mark-read when messages load in ChatArea
- Unread dot for channels never opened

Also: favicon/icon refresh in index.html

internal/push/handlers.go

@@ -212,8 +212,10 @@ func (h *Handler) SendChannelNotification(ctx context.Context, channelID, author
 		SELECT DISTINCT ps.user_id, ps.endpoint, ps.p256dh, ps.auth
 		FROM push_subscriptions ps
 		JOIN members m ON m.user_id = ps.user_id
+		LEFT JOIN notification_settings ns ON ns.user_id = ps.user_id AND ns.channel_id = $3
 		WHERE m.server_id = $1 AND ps.user_id != $2
-	`, serverID, authorID)
+		  AND (ns.level IS NULL OR ns.level = 'all')
+	`, serverID, authorID, channelID)
 	if err != nil {
 		h.logger.Error("failed to query push subscriptions", "error", err)
 		return