Phase 2: Move viewer and repo settings to useViewerPermissions

[jennings]

Replace store-held viewer + repo settings with the useViewerPermissions hook added in Phase 0.

Fields removed from store

• viewerPermission
• viewerCanMergeAsAdmin
• repoAllowMergeCommit
• repoAllowSquashMerge
• repoAllowRebaseMerge
• repoHasMergeQueue
• prInMergeQueue

Setters removed

• setViewerPermission
• Any related setters used only by loadOverviewData.

Consumer changes

• pr-overview.tsx, pr-review.tsx, pr-edit-dialog.tsx, and any merge-related UI switch from usePRReviewSelector(...) to useViewerPermissions(owner, repo, prNumber).

Tests

• Verify components render correctly with isPending, error, and success states from useViewerPermissions.
• Delete the store tests for the removed setters.
• End-to-end style: merge-related UI (pr-overview.tsx) renders correctly with mocked viewer permissions.

Part of #351

Depends on #352.