feat: make OG image appearance configurable via theme/config

[rohilsurana]

Problem

`src/server/routes/og.tsx` has all visual properties hardcoded:

• Background: `#0a0a0a`
• Text color: `#fafafa`
• Font: Inter (fetched from Google)
• Title size: 56px
• Dimensions: 1200x630

Site owners using a brand with different colors/fonts get generic-looking social previews.

Also, the output is SVG which many social platforms (Twitter/X, LinkedIn) don't render — should convert to PNG.

Suggested Fix

• Read colors/font from theme config or a new `og` config section
• Convert SVG → PNG using Sharp (already a dependency)
• Cache generated images with content-based keys