feat(editor): add C# (.cs) syntax highlighting

Map the .cs extension to the built-in csharp StreamLanguage from
@codemirror/legacy-modes/mode/clike (already a dependency, no new packages),
so .cs files open with syntax highlighting in the sidebar editor.

tests/unit.spec.ts

@@ -1102,6 +1102,7 @@ describe('editor language mapping', () => {
     expect(languageKeyForExt('yaml')).toBe('yaml')
     expect(languageKeyForExt('sh')).toBe('shell')
     expect(languageKeyForExt('md')).toBe('md')
+    expect(languageKeyForExt('cs')).toBe('csharp')
     expect(languageKeyForExt('txt')).toBeNull()
     expect(languageKeyForExt('log')).toBeNull()
     expect(languageKeyForExt('')).toBeNull()