Estimate preview back button now preserves sub-phase context

Clicking back on contents sub-phase estimates no longer crashes—the sub-phase context is preserved.

Bug Fixes

Clicking the back arrow on estimate previews now correctly preserves the sub-phase context for contents phases. Previously, the back link would drop the sp query parameter from the URL, causing the estimate tab to load without the required sub-phase context—resulting in a render crash with "This page couldn't load." The back button now round-trips the sub-phase so the estimate tab loads cleanly.