refactor(watchlist): extract shared useWatchlistMinerStats hook

[Khaostica]

Summary

WatchlistPage and MinersList each ran the same useAllMiners() → mapAllMinersToStats(...) → filter watched githubIds → map(isEligible: OSS || Discoveries) pipeline in a local useMemo. Hoist that into a single useWatchlistMinerStats(minerIds) hook so the page-level sidebar Miners Activity widgets and the table/cards row data share one source.

No behavior change — MinersList already passed through isLoading and the raw allMinersData for its "unresolved miners" warning, both of which the new hook returns alongside the mapped miners array.

Context: I opened this PR originally to fix inflated PR/Issue dot counts on the watchlist Miners tab — the /miners rollup was double-counting per-(uid, hotkey, repo) miner_evaluations rows for any githubId with multiple registrations. That has since been fixed server-side (the test-API rollup now returns the deduped numbers), so the recompute-from-/prs-and-mirror-/issues-feeds part is no longer needed. What's left is the small refactor that fell out of the original change.

Related Issues

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Screenshots

Not applicable — no rendered output changes. The watchlist Miners tab renders the same numbers from the same data source as before, just from a shared hook.

Checklist

• New components are modularized/separated where sensible
• Uses predefined theme (e.g. no hardcoded colors)
• Responsive/mobile checked
• Tested against the test API
• npm run format and npm run lint:fix have been run
• npm run build passes
• Screenshots included for any UI/visual changes