feat(serve): fleet overview in hardbox serve dashboard

[jackby03]

Summary

Extend the hardbox serve dashboard to render an aggregate fleet overview when JSON reports from hardbox fleet audit are present in the reports directory. No backend required — reads from local files.

Fleet view features

• Host table — one row per host: hostname, last audit timestamp, compliance score, score delta vs previous run, regression indicator
• Per-host drill-down — click a host to see its individual findings (reuse existing single-report view)
• Score timeline — sparkline of compliance score over time per host, rendered from historical JSON files
• Regression badge — visual indicator on hosts whose score dropped since last run

Detection logic

The dashboard auto-detects fleet reports by inspecting the JSON structure:

• Single-host report: { "host": "...", "findings": [...] }
• Fleet report: { "hosts": [ {"host": "...", "findings": [...]}, ... ] }

When fleet reports are present, the landing page shows the fleet overview instead of the single-report list.

Acceptance criteria

• Fleet overview renders when fleet JSON reports are present in --reports-dir
• Host table shows: hostname, score, delta, last audit time
• Per-host drill-down navigates to existing single-host findings view
• Score timeline sparkline rendered per host from historical data
• Graceful fallback to single-report view when no fleet reports found
• No external dependencies — all assets embedded, no JS framework CDN calls
• Tests cover: fleet report detection, score delta calculation
• Docs: docs/SERVE.md updated with fleet overview section