Improve blog index: year/month grouping with two-column issue-list layout

Restructure the flat 27-post blog list into a scannable, time-organized
layout using pure typographic hierarchy (no cards, icons, or gradients):

- Add year overline heading (accent-bright mono, bordered) for top-level grouping
- Add month labels (muted uppercase) for secondary grouping
- Convert post rows from stacked blocks to two-column flex layout:
  narrow mono date column (56px) on the left, content on the right
- Tighter type scale: titles 16px/510, excerpts 14px, dates 12px
- Month separators create visual rhythm without decorative elements
- Responsive: flex-direction column on mobile, date stacks above content
- Maintains all existing post links, tags, and hover behavior

Author: cappy-dev