fix(ui): content-shaped loadingSkeleton (#793) adopted in only 1 of several list/table StateBoundary sites, causing layout jump

[JSONbored]

Context

state-views.tsx:258-260's doc comment explains the loadingSkeleton prop exists specifically to avoid a layout jump when a list/table's content mounts after a generic spinner. apps/loopover-ui/src/routes/app.runs.tsx:251,612-620 passes a custom RunsListSkeleton and gets this benefit. Structurally similar list/table dashboards don't: components/site/dead-letter-queue-panel.tsx, components/site/app-panels/maintainer-panel.tsx (which drives ContributorQualityTable), and miner-panel.tsx all fall through to the generic centered spinner (LoadingState) instead.

Requirements

• Build a content-shaped skeleton for each of the 3 named components' actual table/list shape (following RunsListSkeleton's approach: approximate the real layout's row/column structure so the transition from loading to loaded doesn't visibly jump), and pass it as loadingSkeleton to their StateBoundary.
• Do not change the generic LoadingState component itself — it's still correct for boundaries that don't have a fixed content shape.
• This is a UI-only change under apps/**.

Deliverables

• dead-letter-queue-panel.tsx uses a content-shaped loading skeleton.
• maintainer-panel.tsx (for ContributorQualityTable) uses a content-shaped loading skeleton.
• miner-panel.tsx uses a content-shaped loading skeleton.

Expected Outcome

Loading these 3 dashboards no longer produces a visible layout jump when data arrives, matching app.runs.tsx's existing UX.

Links & Resources

• apps/loopover-ui/src/routes/app.runs.tsx:251,612-620 (RunsListSkeleton, the pattern to follow)
• apps/loopover-ui/src/components/site/app-panels/state-views.tsx:258-260
• components/site/dead-letter-queue-panel.tsx, components/site/app-panels/maintainer-panel.tsx, miner-panel.tsx

[linear-code]

JSO-4287