feat: add API key validation flow for AI provider config

- Add POST /api/providers/validate backend endpoint that sends a test
  request to the provider's chat/completions API to verify the key
- Add validateProvider to frontend API client
- Redesign PanelProviders: show token input inline with Validate button,
  display valid/invalid badge after validation, Save only appears after
  successful validation
- Add i18n keys (EN/FR) for validation flow

💾 Generated with Crush

Assisted-by: GLM-5-Turbo via Crush <crush@charm.land>

web/src/styles/global.css

@@ -475,6 +475,15 @@ input::placeholder { color: var(--text-disabled); }
 .provider-card-meta { display: flex; gap: 16px; font-size: 12px; color: var(--text-tertiary); font-family: var(--font-mono); margin-top: 8px; }
 .provider-card-form { margin-top: 12px; padding-top: 12px; border-top: 1px solid var(--border); }
 
+.provider-setup-hint {
+  font-size: 13px; color: var(--text-tertiary); margin-bottom: 16px;
+  padding: 10px 14px; border-radius: var(--radius); background: var(--bg-surface);
+  border-left: 3px solid var(--accent-dim);
+}
+.provider-setup-token-row { display: flex; gap: 12px; align-items: flex-end; }
+.provider-setup-token-input { flex: 1; }
+.provider-setup-token-actions { display: flex; gap: 8px; flex-shrink: 0; padding-bottom: 1px; }
+
 .config-update-controls {
   display: flex; align-items: center; justify-content: space-between; gap: 16px; flex-wrap: wrap;
 }