Use case-insensitive matching for Git error message "Not a valid object name"

## Summary

Git is changing the capitalization of its `fatal: Not a valid object name` error message to lowercase (`fatal: not a valid object name`) to follow Git's [CodingGuidelines](https://github.com/git/git/blob/master/Documentation/CodingGuidelines#L410-L416), which state that error messages should begin with a lowercase letter.

This change is under review on the Git mailing list:
https://lore.kernel.org/git/pull.2052.git.1771836302101.gitgitgadget@gmail.com

## Problem

This repo currently uses **case-sensitive** string matching to detect this Git error, which will break when Git ships the updated message.

### Affected files

- **preview/git.js** — \`stderrBuffer.includes("Not a valid object name")\` and \`err.message.includes("Not a valid object name")\`

## Suggested fix

Use case-insensitive comparison, for example:
\`\`\`javascript
stderrBuffer.toLowerCase().includes("not a valid object name")
\`\`\`

This will work with both the current and future Git versions.

## Timeline

The Git change is currently under review and will likely be included in a future Git release. Fixing the matching now ensures forward compatibility.