feat(viewer): restyle the session compare page (#1328) (#1331)

* feat(viewer): restyle the session compare page (#1328)

Give the four outcome buckets distinct tones and render Existing Code and Suggested Change side by side on wide screens, scoped to the compare page only. See #1328.

* fix(viewer): address review feedback on compare buckets

Match the Not reviewed bucket title explicitly with a neutral fallback for unknown titles, and add a dark-mode override so the not-reviewed dot and border stay visible.

* refactor(viewer): derive compare bucket tones from shared tokens

The shared design foundation landed the severity and accent tokens; derive the compare outcome tones from them so light and dark adapt automatically, replacing the local hardcoded palette.