feat(security): add Content-Security-Policy headers with strict script-src

Eliminate all inline scripts to enable script-src 'self' without
'unsafe-inline', providing strong XSS mitigation via CSP.

Inline script removal:
- login.phtml: replace inline APP_ASSET_BASE with data-asset-base
  attribute + app-boot.js (matching default.phtml pattern)
- api-docs: extract SwaggerUI init to js/pages/admin-api-docs-index.js
- docs: extract checkbox enablement to js/pages/admin-docs-index.js
- language-switcher: replace onchange handler with data-auto-submit
  attribute + js/components/app-auto-submit.js event listener

CSP policy (dev + prod nginx):
  default-src 'none'; script-src 'self'; style-src 'self' 'unsafe-inline';
  img-src 'self' data:; font-src 'self'; connect-src 'self';
  form-action 'self'; base-uri 'self'; frame-ancestors 'none';
  manifest-src 'self'

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/js/components/app-auto-submit.js

@@ -0,0 +1,12 @@
+/**
+ * Auto-submit: submits the parent form when a [data-auto-submit] element changes.
+ * Replaces inline onchange="this.form.submit()" handlers for CSP compliance.
+ */
+document.querySelectorAll('[data-auto-submit]').forEach((element) => {
+  element.addEventListener('change', () => {
+    const form = element.closest('form');
+    if (form) {
+      form.submit();
+    }
+  });
+});