PHP app: auto-detect BASE_URL so CSS/assets load on any cPanel layout

Root cause of the "site loads unstyled / CSS 404" report: BASE_URL had
to be set manually, so /assets/css/style.css 404'd whenever the cPanel
document root differed from the .env value (subfolder installs, or wrong
BASE_URL).

app/bootstrap.php: when .env BASE_URL is empty, derive it from
SCRIPT_NAME (dirname, strip a trailing /public, normalize). Resolves
correctly for all three layouts: doc root = project root, doc root =
public/, and subfolder install. Explicit BASE_URL still overrides; an
explicit "/" is normalized to empty to avoid //asset double slashes.

README: BASE_URL note updated (auto-detected; leave empty), plus a
"Pages load but look unstyled (CSS/assets 404)" troubleshooting section.
Noted the app ships no JS — design is the single self-contained style.css.

No DB logic, routes, or markup rebuilt — only asset/path integration.
Re-packaged exports/model-directory-php-app.zip. Architect review: PASS.
