Add a /stats/ page

## Background

The repository already exposes a rich amount of structured metadata through attributes such as `@[category]`, `@[AMS]`, and `@[formal_proof]`, all of which are surfaced in `site/data/conjectures.json` via `lake exe extract_names`.

This makes it possible to generate useful repository-wide statistics directly at build time, without introducing new metadata systems or additional dependencies.

## Proposal

Add a `/stats/` page to the website, linked from the top navigation alongside Browse / About / Contribute.

Potential sections could include:

- subject × status tables,
- solved/formalization ratios by field,
- repository health metrics,
- contributor statistics,
- time evolution of additions and solved problems.

Examples of potentially useful views:

- “what is open vs solved in Number Theory?”,
- formalization coverage by AMS area,
- missing or inconsistent metadata,
- active source collections or contributors.

All of this could be generated statically from the existing `conjectures.json` pipeline and integrated into the current `site/build.js` flow, preserving the current lightweight build approach (no backend and no client-side dependencies required).

I can open this incrementally as a sequence of small PRs so that each addition remains independently reviewable. I will start by opening PR 1 as a concrete illustration of the direction.