feat(07-05): frontend AI config client helpers + AdminAiConfigTab system section + Vitest tests
- Add getAiConfig/saveAiConfig/testAiConnection to frontend/src/api/client.js
- AdminAiConfigTab.vue: add System AI Providers section above existing per-user table
- Per-provider accordion (10 providers from PROVIDER_DEFAULTS)
- Write-only API key field (never pre-filled), base URL, model, context_chars inputs
- Set Active (atomic flip), Save, Test Connection buttons with inline badges
- Existing per-user ai-config table and saveConfig logic untouched (Pitfall 6)
- Create frontend/tests/api.spec.js: 4 Vitest tests for getAiConfig/saveAiConfig/testAiConnection
- All 127 frontend tests pass, build exits 0 (D-11 frontend coverage)