Expose original-commit private review in MR screens #642

Expose MR commit browsing and fixed original diff review using captured account-bound controllers and identifier-only routes. Preserve validated literal row identity in the shared diff renderer; connect single-line/range selection to guarded private commit draft save and existing inspection recovery. Show unavailable files and unsupported references honestly. Add English-source localization in all five locales and test responsive light/dark flows, races and deep links first. No public fallback or automatic publication.