fix: migrate to CSS-based Primer theme provider

[setchy]

Summary

• Migrate production and visual-test theme providers to @primer/react/next, aligning the appearance hook, public props type, and setter mock.
• Preserve the existing provider hierarchy, BaseStyles, CSS wrapper attributes, theme mappings, settings, and native bridge behavior. Production changes are imports only; the visual helper changes only its provider import.
• Add four focused real-provider cases to the existing src/renderer/hooks/useAppearance.test.ts unit-test file:
  • Provider updates and descendant wrapper compatibility.
  • Live light-to-dark settings updates.
  • System-mode OS changes without rewriting the saved preference or forcing a fixed native source.
  • One representative accessibility scheme.
• Retain all existing mocked appearance tests, resetting mock behavior between cases. No separate integration file, test project, or broad duplicate appearance suite is introduced.

Addresses three SonarCloud typescript:S1874 findings in App.tsx: AaAzhot1E7Zk-3Vizzqj, AaAzhot1E7Zk-3Vizzqk, and AaAzhot1E7Zk-3Vizzql. Unrelated SonarCloud findings are outside scope.

Local validation

• pnpm check passed.
• pnpm exec tsc --noEmit --incremental false passed.
• Focused appearance/theme/settings tests: 41 passed.
• Full unit suite: 1,869 tests passed across 186 files.
• pnpm build passed; Vite emitted a non-fatal chunk-size warning.
• Pre-commit formatting, lint, type checks, and changed tests passed.
• Strict OpenSpec validation passed; local planning artifacts were aligned with the reduced test scope.

CI status

For latest commit 9703d4d7:

• Lint and TypeScript/unit-test CI passed.
• Visual Regression passed against the existing baselines. Docker was unavailable locally, so this is the visual verification result. No screenshot baselines were changed.
• CodeQL and GitHub Actions security checks passed.
• SonarQube Cloud Analysis was still running when this description was updated; closure of the three findings is not yet claimed.

No dependency, CSS, persistence, routing, or IPC changes. OpenSpec artifacts remain local, consistent with the repository ignoring openspec/.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud