feat: OpenGraph metadata + auto-generated OG card images

- Per-post OG tags (title, description, image, date) in template head
- Twitter Card support (summary_large_image)
- Auto-generate 1200x630 card images from embedded base PNG
- Title, description, domain, date drawn on image via fogleman/gg
- Fonts embedded via go:embed (OpenSans Bold + Regular)
- Optional og-image: and og-description: front matter overrides
- Layout constants in ogimage.go for easy repositioning
- Generated images cached in content/og/ (gitignored)

👾 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta Code <noreply@letta.com>

.gitignore

@@ -3,3 +3,4 @@
 *.log
 .letta/
 .directory
+/content/og/