feat(editor): allow inline code to coexist with highlight and other marks

[j4rviscmd]

Summary

Allow inline code (<code>) to coexist with highlight (backgroundColor), textColor, bold, italic, underline, and strikethrough marks — matching Notion behaviour.

Key changes:

• Override TipTap Code mark excludes from '_' (all) to '' (none) via Code.extend({ excludes: '' })
• Reorder styleSpecs so code is declared last, placing <code> inside the highlight <span> in the DOM — keeps highlight box at normal text height
• Add CSS rules for code-inside-highlight: semi-transparent code background (light), dark opaque background (dark), with subtle ring for visual separation

Related Issue

N/A — standalone enhancement

Type of Change

• ✨ New feature
• 💥 Breaking change
• 🐛 Bug fix
• 📝 Documentation
• ♻️ Refactoring
• 🔧 Chore

Breaking Changes

None

Test

• Run npm run tauri dev
• Select text and apply highlight (backgroundColor)
• Apply inline code to the same text — both marks coexist
• Verify highlight height matches regular highlighted text
• Check code background is visible inside highlight (light mode)
• Switch to dark mode — verify code background is distinct
• Test other mark combinations: code + bold, code + italic, code + textColor

Checklist

• Code follows clean architecture (proper separation of concerns)
• Code is simple and concise (no unnecessary complexity)
• Comments are sufficient for non-obvious logic
• No sensitive information included (credentials, secrets, personal data)
• Conventional Commits format followed in commit messages
• Tests added/updated (or N/A for docs/chore)