Fix editor diff gutter layout and Inbox PR overview diff colors

[EricRasputin]

What changed

• In the editor's diff view (the default "(Working Tree)" tab), the +/− marker sits between the line numbers and the code instead of at the far left of the gutter. Changed rows tint their line numbers, removed lines show their old line number, and the two color bars per line are gone, so it matches the unified diff view.
• Diff tabs put the fold arrows first and size the number column to its digits, so each number sits beside its marker. Ordinary editor tabs are unchanged.
• The Inbox pull request overview (total and per-file +N -N, and the per-file bar) uses the diff color tokens, so it follows the selected Diff colors palette.

Why

Follow-up to #707, which shipped in 0.7.1. That PR was only checked in the unified diff view. In the editor diff it reused the git gutter, which is the leftmost one, so the marker ended up two columns away from its line, with a color bar on each side of the untinted line numbers. The Inbox PR overview was the one diff surface still on hardcoded emerald/red.

UI

Before

After

Both are the same file, rendered from the real editor extensions and app CSS in headless Chromium (dark theme, Default palette). No screenshot of the Inbox overview; it is a class swap to the existing tokens.

Notes for review

• The gutter order is set in two places: editorGit registers its marker gutter at low precedence so it lands next to the code, and FileEditor lists foldGutter() before lineNumbers() in diff tabs.
• The number gutter reserves width for the original file's largest line number, so old numbers on removed rows don't clip or shift the gutter while scrolling.
• On changed rows the diff tint replaces the active-line gutter highlight, and the fold arrow takes the diff text color.
• The scrollbar change overview is untouched.
• The Inbox change is its own commit, so it can be split into a separate PR if you'd rather keep this to the editor.

Checklist

• I ran npm run check
• This PR is small and focused
• I did not mix unrelated changes

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