feat: use FileInput DS component for file uploads, fix certs volume perms

- Replace inline FileField and native <input type="file"> with
  FileInput from @cameleer/design-system (drag-and-drop, icons, clear)
- Update CertificatesPage and SsoPage to use FileInput + FormField
- Fix /certs volume permissions (chmod 775) so cameleer user can write

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -51,6 +51,8 @@ services:
         {"subject":"$$SUBJECT","fingerprint":"$$FINGERPRINT","selfSigned":$$SELF_SIGNED,"hasCa":$$HAS_CA,"notBefore":"$$NOT_BEFORE","notAfter":"$$NOT_AFTER"}
         METAEOF
         mkdir -p /certs/staged /certs/prev
+        chmod 775 /certs /certs/staged /certs/prev
+        chmod 660 /certs/*.pem 2>/dev/null || true
     environment:
       PUBLIC_HOST: ${PUBLIC_HOST:-localhost}
       CERT_FILE: ${CERT_FILE:-}