Icon engineering hub + Verge-by-default + layout consistency

Visibility + consistency pass on the design system. (1) **Icon engineering hub** — the `/icons/` page is now an engineering hub: an "Icon engineering" section with cards for Anatomy, Generation (kicon) and Verge Symbols, above the product catalog, so the icon **anatomy** (rendered from KMD via the KDX→HTML backend) and grammar are discoverable instead of buried. (2) **Verge is the default UI style on every variant** — the `/style` gallery marked the OS-native look (gnome/fluent/ macOS/Material/Cupertino) as default per platform; now Verge — Koder's own design language — is the default everywhere, with the OS look-alikes kept as selectable presets. (3) **Layout consistency** — all pages now centre content uniformly (the prior split between ToC and non-ToC pages is gone). (4) The release log itself was caught up (v0.6.0) after a month of uncaptured work.