Add configurable link hover highlighting in editor and preview

[pbek]

Feature request

Add clear hover feedback for links in the note editor and preview.

Desired behavior

• In the note editor, highlight only the actual URL destination of an inline Markdown link, not its label, delimiters, checkbox, or list prefix.
• Show the link hover color and pointing-hand cursor in the editor only while Ctrl is pressed, matching the existing Ctrl+click requirement.
• In the preview, highlight links and show the pointing-hand cursor on ordinary hover because preview links open without Ctrl.
• Use an I-beam cursor over non-link text.
• Add a dedicated Link hover foreground color to editor color schemas, including defaults for the bundled Light and Dark schemas.
• Preserve search, FakeVim, and diagnostic extra selections while displaying hover feedback.

The behavior should work with both QTextBrowser and QLiteHtml preview backends.