An empty diff says "No changes." in one plain muted line, not a boxed card #10046

For: an empty Files tab reads quietly, like the cut-diff line from #9988.

Done when:

• [x] In src/web/diff-view.ts, renderDiff returns a muted line instead of <div class="card empty">No changes.</div> when a diff has no files. It uses the plain empty block that #10017 brings in if that has landed, and a muted <p> if not.
• [x] A test asserts that the empty diff has no card class and still says "No changes."
• [x] bun scripts/design-lint.ts and bun test pass.

Files: src/web/diff-view.ts, a test.

Reviewed pull #10063 at its head: ticked In src/web/diff-view.ts, renderDiff returns a muted line instead of <div class="card empty">No changes.</div> when a diff has no files. It uses the plain empty block that #10017 brings in if that has landed, and a muted <p> if not.; A test asserts that the empty diff has no card class and still says "No changes."; bun scripts/design-lint.ts and bun test pass..