feat: add Ctrl+Alt+H keyboard shortcut to toggle highlight color

[MarsEB71]

Summary

• Adds Ctrl+Alt+H shortcut to the document editor toolbar
• First press applies the current highlight color to selected text
• Second press removes the highlight if the selection's color matches the current highlight color

How it works

Reads the highlight state directly from api.get_TextProps() on each keypress rather than relying on cached button state. Uses _setMarkerColor('transparent') (which calls SetMarkerFormat(true, false)) to remove the highlight — not SetMarkerFormat(false) which only exits painting mode without removing existing highlights.

Test plan

• Select text → Ctrl+Alt+H → text highlighted in yellow
• With highlighted text selected → Ctrl+Alt+H → highlight removed
• Change highlight color via toolbar → Ctrl+Alt+H → applies new color
• No text selected → Ctrl+Alt+H → no crash

Closes #55

🤖 Generated with Claude Code

[moodyjmz]

Not sure if this gets added, I can't decide that unilaterally, however I would give some feedback generally

Why IIFE?
Why place the logic code into the init block?
Would it be better to have the logic as a named method alongside the other named methods?

I see you are using Claude. You can ask it to look at the existing patterns in the file and follow convention. :)