Full diff preview when previewing changes

[k-anshul]

closes https://linear.app/rilldata/issue/ENG-1120/option-to-view-file-changes-before-committing-in-cloud-editing

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[k-anshul]

@codex review

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[AdityaHegde]

1. Lets move the new files to changes folder under project.
2. New components should be moved to svelte5

[AdityaHegde]

1. This was probably in the original component as well, but it doesnt look good in dark mode. Maybe out of scope of this PR.
2. We should add the file icon from left nav beside the files. I dont see a way to configure it in the diff view so lets add it to the sidebar. We can hide the icon in diff view.