FE: Comparison page (picker + summary)

[alanpeixinho]

Summary

Create the comparison view shell: a revision picker for selecting two commits, tab structure, and headline flip counts.

Details

Revision picker

• User manually selects Side A and Side B commits on the same tree/branch
• Reuse useCommits hook (same data feeding the commit navigation graph) to populate available commits
• No auto-selection — user always picks explicitly

Tab structure

• Same as current Tree Details: Builds | Boots | Tests
• Each tab shows its own headline numbers

Headline summary

• Per tab: "N regressions (PASS→FAIL), M fixes (FAIL→PASS)"
• Transitions involving INCONCLUSIVE or a null side are visible in the table but not in headline counts

Empty / edge states

• When no rows differ between the two commits, show a clear "No changes between these commits" state
• When a selected commit has no data at all (every row null on that side), surface it clearly rather than showing an all-null table

Implementation notes

• Can be built with a mocked API response until the BE endpoint (BE: Compare endpoint for builds, boots, and tests #2009) lands
• Borrow patterns from Hardware Details multi-commit selection (treeCommits URL param, navigateToThePast callback)
• Entry point TBD: new route vs query param on existing Tree Details route (resolved together with the URL state issue)