feat(preview): footnote hover preview

Hovering a footnote reference (rendered by marked-footnote as
<a data-footnote-ref href="#footnote-N">) now shows a small popover
with the footnote body text, matching the UX of Typora and Obsidian.

- src/renderer/footnote-preview.js — pure DOM module. Mounts a single
  popover once per preview pane; mouseover delegates via Element.closest()
  to the ref, lookups the matching <li id="footnote-N"> in the same pane,
  strips the backref ↩, and positions above/below the cursor with edge
  clamping. CSS.escape() fallback for non-browser environments.
- src/renderer.js — _renderPreview calls mountFootnotePreview once per
  pane (gated by a dataset marker so re-renders don't accumulate
  listeners).
- eslint.config.js — CSS + Element added to browser globals.

Tests (8 new, tests/footnote-preview.test.js): mount/unmount, delay +
timer behavior, mouseover/mouseout show/hide, dangling ref graceful
no-op, backref ↩ stripped from the displayed text, cancel-pending-show
when a new ref is hovered, idempotent remount guard.

Full suite: 69 suites, 797 tests, lint+format clean.

Amit Haridas

eslint.config.js

@@ -62,6 +62,8 @@ module.exports = [
         HTMLInputElement: 'readonly',
         HTMLTextAreaElement: 'readonly',
         getComputedStyle: 'readonly',
+        CSS: 'readonly',
+        Element: 'readonly',
         // Electron
         electronAPI: 'readonly',
         // Libraries