History tab: keep top-level history UI and add scan-to-scan compare view

[techmore]

Description

The History tab is now a top-level surface and already shows persisted scan history plus diff summaries. The remaining work is to add a true compare-any-two-scans workflow and richer historical detail navigation.

What Is Already Done

• History is promoted to a top-level navbar tab.
• Historical scans load from /api/scans without relying on transient button state.
• Diff summaries are persisted and rendered in the History UI.
• Changed-scan filtering exists.

Remaining Scope

• Allow selecting any two scans for the same customer/target and viewing a direct comparison.
• Add a more detailed historical scan detail view beyond the summary cards.
• Keep the existing diff-summary cards as the lightweight overview.

Why This Still Matters

The main history surface exists now, but comparison is still limited to automatic previous-scan diffs. Operators still need an explicit compare workflow for arbitrary scan pairs.

Priority

Medium-High

Implementation Notes

• Build on the existing persisted diff_summary and /api/scans metadata.
• Add regression coverage for selecting and comparing two saved scans.

[techmore]

Implementing the remaining compare workflow validation now: adding browser-level coverage for selecting a base scan and comparing two historical scans, plus stricter API validation for mismatched comparisons. I will close this once the full History compare flow is covered and green.

[techmore]

Implemented on codex/layout-audit in 7689d13.

Delivered scope:

• Added authenticated /api/scans/compare for explicit scan-pair comparison.
• Added History compare UI with base-scan selection, compare actions, and a dedicated comparison panel.
• Added richer per-scan detail blocks in History cards.
• Added route-level regression coverage for explicit diff results plus mismatched target/customer validation.
• Added browser-level regression coverage for selecting two historical scans and rendering the comparison result.

Validation:

• ./.venv/bin/python -m pytest -q
• Result: 183 passed, 6 skipped