fix(frontend): show security audit menu in simple mode

The security audit group (Risk Control + Prompt Audit) was hidden from
the sidebar via `hideInSimpleMode`, but nothing else in the stack
restricts it in simple mode:

- `router/index.ts` simple-mode `restrictedPaths` does not cover
  `/admin/risk-control` or `/admin/prompt-audit`
- the `/risk-control` and `/prompt-audit` admin route groups have no
  `RunMode` gate, and neither does `internal/securityaudit/` nor
  `service/content_moderation.go`
- `SettingsView.vue` renders the risk control toggle together with a
  `<router-link to="/admin/risk-control">` shortcut, and the settings
  page stays visible in simple mode

The feature is therefore fully usable in simple mode and already
reachable through the settings page — only the sidebar entry was
missing. Drop the flag so the menu matches actual behaviour.

The group remains gated by `risk_control_enabled` (opt-in, default
false) through `featureFlag: flagRiskControl`.

frontend/src/components/layout/AppSidebar.vue

@@ -775,7 +775,6 @@ const adminNavItems = computed((): NavItem[] => {
       path: '/admin/security-audit',
       label: t('nav.securityAudit'),
       icon: ShieldIcon,
-      hideInSimpleMode: true,
       expandOnly: true,
       featureFlag: flagRiskControl,
       children: [