fix(miners): Score sort matches displayed value and Repository is sortable

[eureka0928]

Summary

On the Miner Details page's PR table (/miners/details?githubId=<id>&tab=pull-requests):

• Score column sort was broken for open PRs. The comparator read pr.score unconditionally while the cell's render fallback shows collateralScore for open PRs, score for merged PRs, and - for closed-unmerged. Every open PR collapsed to sort key 0 and clicking Score never reordered them. Verified against miner 42954461: 17 open PRs, all pr.score = "0.000000", collateralScore ranging from 0.001 to 0.370 → old comparator had 1 distinct sort key, new comparator has 17.
• Repository column was not sortable. The header was a plain <TableCell>Repository</TableCell> with no click target, even though the other five columns are sortable.

Changes — src/components/miners/MinerPRsTable.tsx

1. getEffectiveScore(pr) helper mirrors the Score cell's render fallback exactly (CLOSED && !mergedAt → 0, !mergedAt && collateralScore → parseFloat(collateralScore), else parseFloat(score)). Used in the 'score' sort case so the sort key matches the displayed value.
2. 'repository' added to PrSortField with a.repository.localeCompare(b.repository) and a pullRequestNumber tiebreaker. Within a single repo, rows get a deterministic order that flips with the primary sort direction.
3. DEFAULT_SORT_DIR: Record<PrSortField, SortDir> map centralizes each column's natural idle direction (string columns ascending, numeric/date descending). handleSort reads from it instead of branching on the field name, so adding Title/Author sort in the future is a one-line map entry.
4. Repository header wrapped in <TableSortLabel> consistent with PR #, +/-, Score, Date. Idle-state direction pulled from DEFAULT_SORT_DIR.repository.

Diff stat: 1 file, +40 / −4.

Related Issues

Closes #276

Type of Change

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

Screenshots

Before:

Screencast.from.2026-04-15.01-53-43.webm

Clicking Score on miner 42954461 leaves the open-PR rows (marked "Collateral") in API order. Repository header is static.

After:

Screencast.from.2026-04-15.01-51-27.webm

Same miner, same clicks. Score now reorders open-PR rows by their displayed collateral value (highest-first, then lowest-first on toggle). Repository is a sortable header that groups rows by owner/repo with a pullRequestNumber tiebreaker.

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