feat: add visual avatar picker for equipping new relics and light cones

- replace the ~100-option avatar <select>s in Add Relic / Add Light Cone with a searchable picker modal (keep-in-inventory card + avatar cards with element/path/level)
- warn when the chosen character already wears a relic in that slot or a light cone
- click+Confirm or double-click applies; Esc closes only the picker; search autofocused
- localize all new strings (th/en, other locales fall back to en) and refresh chips/picker on language change
- add AGENTS.md with localization and cache-busting rules
- bump app.js v3.20, styles.css v2.29, i18n.js v2.17 and align all i18n imports

js/item-catalog.js

@@ -1,4 +1,4 @@
-import { getCurrentLanguage, t, translatePath } from './i18n.js?v=2.16';
+import { getCurrentLanguage, t, translatePath } from './i18n.js?v=2.17';
 
 const CATALOG_URL = new URL('../assets/item-catalog.json?v=2', import.meta.url);
 const PAGE_SIZE = 48;