Make PlantUML diagrams readable on dark themes

[ckorhonen]

Summary

• add a PlantUML-specific image renderer that marks generated diagram images with plantuml-diagram
• give only those generated diagram images a white background in the markdown preview
• leave normal markdown images unchanged

Fixes #2559

Validation

• git diff --check
• parsed browser/lib/markdown.js with @babel/parser
• NODE_PATH=/private/tmp/boostnote-eslint/node_modules /private/tmp/boostnote-eslint/node_modules/.bin/eslint browser/lib/markdown.js
• isolated markdown-it render check confirming PlantUML output includes class="plantuml-diagram" and preserves alt text
• isolated Stylus snippet check confirming the nested selector compiles to img.plantuml-diagram { background-color: #fff; }

Note: full dependency install is currently blocked by the legacy git://github.com/adobe-webplatform/eve.git dependency resolving to a missing commit, so I could not run the full app test suite.