[Feature Request]: Highlights overview section in the page panel (TOC-style)

[perber]

Context

Follow-up to Discussion #1354. The base request from that discussion — ==highlighted== markdown syntax rendering as <mark> plus an editor toolbar control — already shipped in #1421.

Still open is the second part:

Cherry on the cake would be an overview of all highlighted positions in the current article, probably implemented like the TOC.

Proposal

Add a "Highlights" section to the existing right-hand page panel (TocSidePanel / TocDropdownButton), listing every ==highlight== span on the current page and letting the reader jump to it.

This reuses the panel that already hosts On this page (headings) and Attached Media (attachments) — highlights become a third section in the same shared component, exactly the way attachments were added.

Behaviour

• Collect all highlight spans from the current page, in document order.
• Render them as a new section in the shared panel, using each highlight's text as the label (truncated for long spans).
• Clicking an entry scrolls the highlight into view and briefly emphasises it so it's findable.
• The section is always available — it simply renders only when the page contains at least one highlight. No setting/toggle.
• showRightPane in PageViewer.tsx must also become true when highlights exist (currently showTocButton || attachments.length > 0).
• Respect the existing "code fences / inline code are not markers" rule — the overview must not list == occurrences inside code.

Implementation sketch

• Extraction: new helper next to extractTocEntries.ts, reusing its code-fence-skipping logic, returning the highlight spans (text + anchor id). Alternatively derive from the parsed tree in MarkdownPreview.tsx where remark-flexible-markers already runs.
• Anchors: <mark> elements currently carry no id. Assign stable ids via a small rehype step (like rehypeLineNumber does for headings), or reuse the existing data-line scroll mechanism, so entries can link to a target.
• UI: extend TocSidePanel.tsx and TocDropdownButton.tsx with the new section; wire the entries through PageViewer.tsx next to tocEntries / attachments.
• Scroll spy: optional — reuse useTocScrollSpy so the active highlight is marked while scrolling, consistent with headings.
• i18n: new key(s) in the viewer namespace (toc.highlights or similar), en/ + de/, key-parity test enforced.
• Tests: Vitest unit tests for the extractor (document order, code-fence exclusion, nested formatting inside a highlight e.g. ==**bold**==, empty page → no entries) and a component test for click-to-scroll and for the section being hidden when there are no highlights.

Open questions

• Exact scroll-into-view emphasis (reuse the heading flash, or a dedicated mark pulse?).