feat: library statistics dashboard

[nikazzio]

Summary

Add a statistics overview to the Library view showing aggregate metrics about the user's manuscript collection.

Motivation

Users accumulate manuscripts over time but have no at-a-glance view of their archive's scope. A dashboard gives a sense of completeness and helps prioritize transcription work.

Proposed metrics

• Total manuscripts in library
• Total pages across all manuscripts
• Pages with transcriptions (count + %)
• Pages with OCR completed (count + %)
• Total disk space used (downloads + thumbnails)
• Provider breakdown (pie/bar chart: how many from BnF, BSB, etc.)
• Recently added / recently worked on

Proposed approach

• New route GET /library/stats returning an HTMX partial.
• All metrics computable from SQLite queries on existing tables.
• Optional: lightweight chart via Chart.js or pure CSS bars.
• Accessible from Library toolbar or as a collapsible panel.

Acceptance criteria

• Stats panel visible in Library view
• Shows manuscript count, page count, transcription coverage
• Shows disk usage
• Provider distribution displayed
• Responsive — works on narrow screens
• Data loads fast (<500ms for typical library of ~100 manuscripts)

Technical notes

• Disk usage: sum file sizes under downloads/ per manifest.
• Transcription coverage: query transcriptions table grouped by manifest.
• Consider caching stats with a short TTL to avoid re-scanning on every load.