bridge: fix doubled bin/ in bundled-mode frankenphp path resolution

resolveFrankenphpBin() returned `applicationDirPath() + "/bin/frankenphp"`,
but applicationDirPath() inside an AppImage is already `usr/bin/` —
where the host binary itself runs from. The "/bin/" prefix produced
the doubled path, e.g.:

  /tmp/appimage_extracted_<hash>/usr/bin/bin/frankenphp

…which doesn't exist. The supervisor logged the lookup failure and
kept retrying, eventually hitting the perfsmoke 8s cold-start
deadline.

build-appimage.sh:148 installs frankenphp at `usr/bin/frankenphp`
(sibling of the host binary, per the layout comment at line 18). The
fix is to drop the spurious `/bin/`. Other resolvers in the same
file (resolveSymfonyDir, resolveCaddyfilePath) already use the
correct `here + "/<file>"` pattern.

Bug shipped since the bundled-mode supervisor was added — would
have hit anyone running the AppImage. Local `make quality` only
exercises dev mode (BRIDGE_URL set), so the integration test loop
never reached this codepath; CI's perfsmoke against the actual
AppImage is the only thing that catches it. Manual test would be:
launch the AppImage with BRIDGE_FRANKENPHP_BIN unset and watch the
phpqml.bridge.bundled log.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

framework/qml/src/BackendConnection.cpp

@@ -135,7 +135,8 @@ QString BackendConnection::resolveFrankenphpBin() const
 {
     const QByteArray override = qgetenv("BRIDGE_FRANKENPHP_BIN");
     if (!override.isEmpty()) return QString::fromUtf8(override);
-    return QCoreApplication::applicationDirPath() + QStringLiteral("/bin/frankenphp");
+    // build-appimage.sh installs frankenphp as a sibling of the host binary at usr/bin/frankenphp.
+    return QCoreApplication::applicationDirPath() + QStringLiteral("/frankenphp");
 }
 
 QString BackendConnection::resolveSymfonyDir() const