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/markup/html2markdown.ts

@@ -86,8 +86,8 @@ function prepareProcessors(ctx:ProcessorContext): Processors {
 
 function processElement(ctx :ProcessorContext, processors: Processors, el: HTMLElement): string | void {
   if (el.hasAttribute('data-markdown-generated-content')) return el.textContent;
-  if (el.tagName === 'A' && el.children.length === 1 && el.children[0].tagName === 'IMG') {
-    return processElement(ctx, processors, el.children[0] as HTMLElement);
+  if (el.tagName === 'A' && el.children.length === 1 && el.firstElementChild!.tagName === 'IMG') {
+    return processElement(ctx, processors, el.firstElementChild as HTMLElement);
   }
 
   const isListContainer = el.tagName === 'OL' || el.tagName === 'UL';

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