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)

frontend/src/api/client.js

@@ -290,6 +290,27 @@ export function adminDeleteUser(id, adminPassword) {
   })
 }
 
+// ── System AI Provider Configuration (D-08, D-15) ───────────────────────────
+
+export function getAiConfig() {
+  return request('/api/admin/ai-config', { method: 'GET' })
+}
+
+export function saveAiConfig(body) {
+  return request('/api/admin/ai-config', {
+    method: 'PUT',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify(body),
+  })
+}
+
+export function testAiConnection(providerId) {
+  return request(
+    '/api/admin/ai-config/test-connection?provider_id=' + encodeURIComponent(providerId),
+    { method: 'GET' }
+  )
+}
+
 // ── Folders ───────────────────────────────────────────────────────────────────
 
 export function listFolders(parentId = null) {