docs: more mascot art in the README

[Razee4315]

What

The README had the banner and product screenshots but none of the personality layer. Added, all from the existing asset kit, optimized into images/art/ (12 files, 180 KB total):

• "Why MarkLite?": writing-desk scene floated right of the origin story, plus a one-line nod that the mascot lives in the app
• Feature subsection headings each get their chunky icon (pencil, flask, sparkles, folder, theme swatches, bolt)
• AI assistant section opens with the wizard "messy scribbles to clean text" scene
• Installation and Keyboard Shortcuts headings get the rocket and keyboard icons
• Contributing: hugging mascot floated right
• End of file: sleeping mascot + "~ end of file ~" sign-off, mirroring the website footer

All 22 local image refs verified to exist. No copy changes beyond one added blockquote line.