style: modernize CSS with custom properties, dark mode, and highlight merging

- Rewrite BlazorDiff.css with 22 CSS custom properties for full theming
- Add dark mode support via prefers-color-scheme: dark
- Add word-level highlight classes (inserted-word, modified-word, deleted-word)
- Add character-level highlight classes with stronger backgrounds
- Add adjacent highlight merging via :has() and + sibling selectors
  for smooth pill-shaped highlighting across consecutive characters
- Extract border-radius/padding into --diff-char-radius, --diff-char-padding,
  --diff-word-radius, --diff-word-padding custom properties
- Replace rgba() with color-mix() for CSS variable compatibility
- Add responsive breakpoints, print styles, reduced-motion, focus states
- Add stats badge styling system
- Generate matching minified CSS

Author: lzinga