feat(training): trained-weights overview with per-fit reach

[Flowm]

What

Adds two things to the training page (/train), then refactors its layout to match the rest of the app.

1. Device-wide trained-weights overview

A new section at the bottom of the page lists every location with a stored fit on the device (scanning meteocompare:weights:*): name, trained date, out-of-sample improvement, count of models actually tuned, an inline reach selector, jump-to navigation, and Clear. Current location sorts first.

2. Per-fit "reach"

A stored fit can now declare a reach radius (This point only / 25 / 50 / 100 / 250 km) so its trained weights also apply to nearby locations that have no fit of their own.

Resolution lives entirely behind the existing loadWeights(lat, lon) chokepoint:

• the exact grid cell still wins;
• otherwise the nearest training location whose reach covers the point is used;
• uniform within the radius, hard cutoff at the edge (no distance falloff).

Because it's behind loadWeights, the live forecast and the verification "Aggregate (tuned)" row inherit it for free.

3. Collapsible-heading refactor

/train now uses the same CollapsibleSection component as the forecast and verification pages. The page-level <h1>/<h2> give way to section titles; the location and the stored-fit count surface as header summaries while collapsed.

Why

Closes the gap where a fit trained at one location couldn't benefit nearby points, and gives users a single place to see/manage every fit. The refactor unifies heading/collapse behaviour across all three pages.

Notes for reviewers

• Storage shape change: StoredWeights gains optional location: {name, detail?, latitude, longitude} + radiusKm. Pre-existing entries (no location/radius) fall back to their grid-cell center and this point only, so they keep working unchanged.
• Verification behaviour: with reach set, the "Aggregate (tuned)" row will now appear at covered neighbours too (that row ignores the live toggle by design). Intentional, but worth a look if you'd prefer reach to affect only the live forecast.
• Docs: added a Training glossary section to CONTEXT.md (Trained weights / Training location / Reach), deliberately avoiding coverage and region which are already taken. No ADR — treated as an extension of ADR 0007.
• New src/domain/geo.ts haversineKm helper for the radius test.

Verification

• npm run lint, npm run type-check, npm test all green (164 tests, incl. 8 new for reach/precedence/management + 4 for haversine).
• Browser-verified: overview renders/sorts, reach select persists and re-resolves, jump-to navigates, sections collapse/expand with summary chips.

Assisted-by: ClaudeCode:claude-opus-4-8

[github-actions]

⛅ Preview deployment

This PR is live at https://preview-ee636ede-meteocompare.frcy.workers.dev

Commit ee636ede — reproduce locally with npm run deploy:preview.