Fixes #8663

Problem

Highlighting a block (right-click bullet → pick a color) is completely dropped when exporting a page as Text, OPML, or HTML. The highlight is stored as the :logseq.property/background-color block property, but that property is marked :hide? true (so it doesn't clutter the properties UI), and export code filters out all :hide? true properties, dropping the highlight along with it.

Fix

When generating export content, wrap a highlighted block's text in ^^...^^ (Logseq's existing inline highlight markup) if the block has background-color set. This is gated behind a new :encode-highlight-as-mark? option only passed by the actual "Export page" feature, so it does not affect markdown-mirror generation (which shares the same underlying file.cljs code). Text and OPML export then just preserve ^^...^^ as-is (already handled), and HTML export already turns ^^...^^ into a real <mark> tag, so the block renders as visibly highlighted.

Note: this preserves that a block was highlighted, not which of the 7 colors was picked, since ^^text^^ has no color parameter. Capturing the exact color would need a new syntax extension, which felt like a separate, larger discussion.

Testing

Reproduced and verified manually against a fresh DB graph, including two highlighted blocks in a parent/child structure:

• Before fix: none of the 3 export formats showed any trace of the highlight.
• After fix: Text/OPML preserve ^^text^^, HTML renders <mark>text</mark> for both blocks at both nesting levels.
• Added a unit test confirming the markdown-mirror feature is unaffected (it shares the same underlying code but should never encode highlights, since it isn't the "Export page" feature).
• bb dev:lint-and-test: 0 lint errors/warnings, full unit test suite passes (0 failures/errors).