modernize theme to github-style with purple accent
- full CSS rewrite: github primer color tokens, css vars for all colors
- fonts: oxanium (headings), ibm plex sans (ui), jetbrains mono (code)
- layout: flex-column body for proper sticky footer, max-width container
replacing inconsistent 95%/94% widths, float removal from tree icons
- dark mode: all diff colors, badges, surfaces now use vars (no hardcoded)
- ref badges: purple head, yellow tag, green branch with icon fill colors
- table rows: re-enabled alternating rows, hover states, styled headers
- mobile: breakpoints at 768px and 480px, stacked diff view on small screens
- svg icons: updated fills to match new color scheme (light + dark variants)
- html fragments: header role=banner, logo resize, site-desc semantic classes
