feat(ui): make code blocks distinct from comment background

[MattPua]

Problem

Code blocks rendered inside comment/finding bodies blended into the cards wrapping them. The shared CodeBlock primitive used a gray-2 fill — the same color as the PR comment card around it — with a faint gray-4 border, so fenced code blocks were nearly indistinguishable from the surrounding comment.

Changes

Before:

After:

• Bumped the <pre> fill from gray-2 to gray-3 and the border from gray-4 to gray-6, so code blocks read as a distinct, slightly recessed panel.
• Single-file change to the shared CodeBlock primitive — PR comment threads, scout emission cards, and signal cards all benefit at once.
• gray-3/gray-6 are mode-aware Radix steps, so light and dark mode both stay correct.

How did you test this?

• Ran pnpm --filter @posthog/ui typecheck — passes.
• Did not visually verify in the running app.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 5157e02.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(ui): make code blocks distinct from ..." | Re-trigger Greptile}