feedback5 R6: add a correctness-only eslint

Flat config for Node and the browser, with unused names as warnings. The run is clean of errors after three small fixes: the lobby spectate path imports ERR, a disable comment for a rule that is not installed is gone, and two atlas regexes spell the BOM as an escape.

public/js/screens/lobby.js

@@ -10,7 +10,7 @@
 // plays 战场#01, 险境 draws one of 8, 绝境 / 终极 one of 7 (m01 excluded).
 
 import { useEffect, useRef, useState } from '../../vendor/hooks.module.js';
-import { DIFFICULTIES, DIFFICULTY_NAMES, DIFFICULTY_COLORS, ROOM_CODE_LEN, MAX_SEATS, MAX_SPECTATORS, modeIdFor } from '../../../shared/constants.js';
+import { DIFFICULTIES, DIFFICULTY_NAMES, DIFFICULTY_COLORS, ROOM_CODE_LEN, MAX_SEATS, MAX_SPECTATORS, modeIdFor, ERR } from '../../../shared/constants.js';
 import { html, Button, Icon, MicroLabel, Panel, TextField, PingPill, AvatarFrame, Tooltip, Spinner, DifficultyIcon, doctorNo } from '../ui/components.js';
 import { toast, toastError } from '../ui/toasts.js';
 import { GuideButton } from '../ui/guide.js';