Replace the hand-rolled textarea+Prism overlay in CodeEditor with
CodeMirror 6. The overlay hid the textarea selection with
`selection:bg-transparent` and `WebkitTextFillColor: transparent` as a
workaround for wrap drift between the highlight layer and the textarea,
which left Ctrl+A / drag-select / Ctrl+C with no visible result.
CodeMirror handles selection, copy, undo, and wrap natively. Custom
themes match the previous prism-react-renderer github (light) and
nightOwl (dark) palettes; background is transparent so the surrounding
bg-muted/30 still shows; font, font-size, line-height, padding, and
gutter-less layout are preserved.
Public CodeEditor props are unchanged (value, onChange, language,
readonly, exactText, className, minHeight, heightMode) so all seven
call sites keep working. The sensitive-key blur is reimplemented as a
CodeMirror Decoration and stays gated behind the existing Eye toggle.
Test setup gets a Range.getClientRects polyfill so CM mounts cleanly in
JSDOM, and shared-page.test.tsx checks .cm-content text directly.
Closes#1247
Tam Nhu Tran