Helmet merges default CSP directives (including `upgrade-insecure-requests`)
into custom directives when `useDefaults` is true (the default). This caused
browsers to upgrade all HTTP sub-resource requests to HTTPS, breaking static
assets when the server runs over plain HTTP.
This commit conditionally sets `upgrade-insecure-requests` based on
FORCE_HTTPS: enabled in production (where HTTPS is available), explicitly
disabled (null) otherwise to prevent browser SSL errors on home servers
and development environments.
Also extracts `shouldForceHttps` to avoid repeated env lookups.
e25fec4e4a