feat(stats): add explicit empty and error states

[emirykl]

Summary

• render a dedicated empty state when no stats are available
• render an accessible error card with a native retry button
• announce fetch-state changes through a polite live region
• keep loading, empty, error, and populated states mutually exclusive
• cover keyboard retry and successful recovery

Test evidence

• npm run lint — passed
• npm test -- --runInBand --no-watchman — 71 suites and 1,224 tests passed
• focused stats suite — 38 tests passed
• impacted StatsClient coverage — 100% statements, branches, functions, and lines
• npm run build — passed
• Prettier and git diff --check — passed

Fixes #519

Verification

• PASS: npm run lint
• PASS: npm test -- --runInBand --no-watchman (71 suites, 1,224 tests)
• PASS: focused stats tests (38 tests)
• PASS: StatsClient coverage (100% statements, branches, functions, and lines)
• PASS: npm run build
• PASS: Prettier check
• PASS: git diff --check

Diagnostics

Recovered command failures: /bin/zsh -lc 'npx prettier --check src/app/stats/Client.tsx src/app/stats/page.test.tsx && npx jest src/app/stats/page.test.tsx --runInBand --no-watchman' (1), /bin/zsh -lc './node_modules/.bin/prettier --check src/app/stats/Client.tsx src/app/stats/page.test.tsx && ./node_modules/.bin/jest src/app/stats/page.test.tsx --runInBand --no-watchman' (1)

[mikewheeleer]

polished work @emirykl, merging it in 🚀