Migrate scripts/check-coverage.js to TypeScript

[shiba4life]

What

Migrates src/server/static-react/scripts/check-coverage.js → check-coverage.ts.

Also adds "scripts/**/*.ts" to the tsconfig.json include array so the scripts directory is covered by npm run typecheck.

Types added

• CoverageMetric — shape of each metric entry ({ total, covered, skipped, pct })
• FileCoverage — per-file coverage object keyed by lines | functions | statements | branches
• CoverageSummary — the full JSON shape (index signature [filePath: string]: FileCoverage plus a total key)
• MetricKey — keyof FileCoverage to tighten the metrics iteration
• All function parameters and return types annotated
• catch (error: unknown) narrowed with error instanceof Error guard

Validation

• Typecheck (npm run typecheck): passes — only pre-existing TS5101 baseUrl deprecated warning present before and after this change.
• Tests: this script has no test file; no test run needed.

Pre-existing failures

TS5101: Option 'baseUrl' is deprecated — present on main before this PR, unrelated to this migration.

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