FE: Comparison details on main AB page

[alanpeixinho]

Summary

On the main AB comparison page (#2010), render row-level diffs under Builds | Boots | Tests — not only headline counts.

Details

• Builds (BE: Compare endpoint — builds #2020): Config, Architecture, Compiler, Side A, Side B
• Boots / Tests (BE: Compare endpoint — boots and tests #2021): Path, Config, Platform, Side A, Side B
• Rows where status_a ≠ status_b only (incl. one-sided null)
• Reuse status badges; null → neutral dash
• Empty state: same "No changes between these commits" as FE: Comparison page (picker + summary) #2010
• Tables live in FE: Comparison page (picker + summary) #2010 shell/tabs (content overlaps FE: Boots/Tests comparison table #2011 / FE: Builds comparison table #2012 — this issue is main-page integration)
• Mock OK until BE lands
• Filter/sort → separate issue