refactor(common): extract shared LoadingCard component

[fansilas]

Summary

Adds a shared <LoadingCard> component at src/components/common/LoadingCard.tsx to replace the inline <Card sx={{ p: 4, textAlign: 'center' }} elevation={0}> + <CircularProgress> pattern that was duplicated across several loading states.

The component accepts an optional size (defaults to 40) and an optional sx that merges over the base Card styles, so callers with custom backgrounds or borders can still use it.

interface LoadingCardProps {
  size?: number;
  sx?: SxProps<Theme>;
}

Replaced inline usages:

• src/components/miners/MinerIdentityRail.tsx (size 32)
• src/components/miners/MinerOpenDiscoveryIssuesByRepo.tsx
• src/components/miners/MinerPRsTable.tsx
• src/components/prs/PRDetailsCard.tsx (custom sx forwarded for background and border)

Sites that wrap the spinner in a <Box> rather than a <Card> are a different layout (centered spinner in a flex container, no card chrome) and were intentionally left alone. The two RepositoryPRsTable / RepositoryIssuesTable loading states render a section header alongside the spinner, so they do not match the spinner-only <LoadingCard> contract and were also left as is to preserve current behavior.

Related Issues

N/A

Type of Change

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

Screenshots

No visual change is expected: the spinner renders at the same size, position, and color on every replaced site.

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