refactor: trim HomePage data layer to what the page actually renders

[cleanjunc]

Summary

HomePage.tsx was calling useDashboardData('35d'), a hook built for the authenticated dashboard. Tracing the destructure, only datasets and isLoading were ever read, and even then datasets.issues.data was never referenced and datasets.repos.data.length was already covered by useStats().uniqueRepositories, which the page also calls. Six memo pipelines inside useDashboardData (kpis, overview, trendLabels, trendSeries, featuredWork, featuredContributors, featuredDiscoveryContributors) ran their full reductions on every dataset change for outputs that never reached HomePage's JSX.

This PR switches the landing page to direct hooks for the data it actually consumes:

• useAllPrs() for the activity feed, the 35 day merge count, and lifetime merged PRs
• useAllMiners() for the top miner rows, miner count, and median merge rate
• useStats() (already used) now also supplies uniqueRepositories, replacing the count previously derived from useReposAndWeights()

useDashboardData stays untouched for DashboardPage, its only remaining caller. Module level EMPTY_PRS and EMPTY_MINERS constants keep useMemo dependency references stable before data lands, avoiding a needless recompute on the first data tick.

Net effect on first paint of the public page: two fewer network requests (/issues and /dash/repos), and three featured pipelines stop running on every dataset refetch.

Related Issues

• Fixes Landing page calls two API hooks and runs three useMemo chains whose results it never reads #974
• Follow up on feat: landing page redesign with auto-rotating panels #950 and feat: redesign landing page for first-time visitors #938, which rebuilt the landing page and introduced the dashboard hook dependency
• Independent of Refactor repository views to avoid global /prs overfetch (use repo-scoped query) #652 (repository view overfetch refactor); both reduce blast radius from useAllPrs() without depending on each other
• Loosely adjacent to Refactor: consolidate loading/error/empty states via QueryBoundary #347 (consolidate loading, error, and empty states via QueryBoundary)
• Does not touch the same surface as the in flight Polish landing page first-contact flow #971 and Polish landing page first-contact flow and proof sections #970, which polish landing page copy and layout

Type of Change

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

Screenshots

Not applicable. No visual change; the rendered tree is identical.

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

[cleanjunc]

@e35ventura @anderdc Could you please review this PR? Thanks!

[anderdc]

Fix conflicts.

[cleanjunc]

Hi @anderdc
I've fixed the conflicts. Could you please review again?
Thank you!