* feat(editor): add multicolor highlight with auto-contrast text
Enable the Tiptap Highlight extension in multicolor mode, replacing the
single-color yellow toggle with a full color picker (16 presets + custom).
When the chosen highlight color is perceptually dark, text inside the mark
automatically renders white for readability.
Changes span the full pipeline:
- Editor: ColorPicker UI, extended renderHTML for contrast detection
- PDF: normalizeMarkElements preserves data-color as inline style
- DOCX: mergeStyle reads actual background-color from <mark>
- Utils: new isDarkColor() luminance helper
Backward-compatible: legacy <mark> without data-color still renders yellow.
Resolves#3109
* fix: handle multicolor highlight edge cases
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Co-authored-by: Amruth Pillai <im.amruth@gmail.com>