feat: support multiple code blocks with independent syntax highlighting

[unclebay143]

Adds support for rendering multiple code blocks with independent syntax highlighting languages.
Fixes #459

C32R50x.mp4

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Changes Summary

• Add multi-block code screenshots: stack up to 4 theme windows in one export, each with its own title, language, and highlighted lines
• Preserve original single-block theme chrome; multi only appends whole windows (no shared fake header)
• Add a floating + on the controls panel edge to add blocks; per-window delete on hover; language/format apply to the active block
• Persist multi-block state in the URL hash (blocks) while keeping legacy single-block params for back-compat
• Always show language labels on multi-block Cloudflare / Trigger.dev windows even when the title is empty

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