3 — Extract shared formatting utilities

Problem: formatDate, formatLastUpdated, formatAuthors duplicated between BlogIndex.vue and BlogByline.vue. formatDate also in ReleaseDownloader.vue.

Action:

• Create .vitepress/data/format.ts with shared functions
• Update all consumers

Files touched:

• .vitepress/data/format.ts (new)
• .vitepress/theme/components/BlogIndex.vue
• .vitepress/theme/components/BlogByline.vue
• .vitepress/theme/components/ReleaseDownloader.vue

Done when: Zero duplicate formatting functions across components.