Adds --number-sections to the pandoc invocation and rewrites the header
CSS so sections are visually separated from their titles:
- TOC gets real disc/circle bullets with nesting
- Each h1 starts with a top border and extra vertical rhythm
- h2 gets its own lighter divider
- Numbered sections (1, 2, 8.1, 8.2, ...) sit in a muted slot ahead of
the title so the eye parses them as metadata, not as part of the
heading text
- TOC has a "Contents" label above the list
- All colors still derived from the app palette (#f5f3ee background,
#2e5b8a accent)