Add estimated read times to posts

## Implementation

Calculate: word count ÷ 200 words/minute

**Display location:**
- Post metadata area (with date/tags)
- Blog index cards

**Format:** "5 min read" or "12 min read"

## Acceptance Criteria
- [ ] Read time calculated correctly
- [ ] Displays on all posts
- [ ] Shows in blog index
- [ ] Styled consistently

**Estimated Time:** 1 hour
