chore: enable unicorn/better-dom-traversing, drop obsolete type stub

- Enable `unicorn/better-dom-traversing` and switch the flagged
  `.children[0]` / `.childNodes[0]` accesses to `.firstElementChild` /
  `.firstChild`.
- Remove the obsolete `eslint-plugin-no-use-extend-native` type stub; it is
  no longer a dependency.
- Drop misleading comments on `no-thenable` (enabled here, not disabled) and
  `try-complexity`.

Assisted-by: claude-code:opus-4.8

web_src/js/modules/errors.ts

@@ -24,7 +24,7 @@ export function showGlobalErrorMessage(msg: string, msgType: Intent = 'error', d
   if (!msgContainer) {
     const el = document.createElement('div');
     el.innerHTML = html`<div class="ui container js-global-error tw-my-[--page-spacing]"><details class="ui ${msgType} message"><summary></summary></details></div>`;
-    msgContainer = el.childNodes[0] as HTMLDivElement;
+    msgContainer = el.firstChild as HTMLDivElement;
   }
 
   // merge duplicated messages into "the message (count)" format