kf6-solid: fix 4 #error stubs + enable UDisks2/UPower backends via D-Bus

The KF6Solid recipe disabled ALL device backends because 4 #error
directives blocked compilation on non-Linux platforms. This meant
zero hardware discovery on Redox.

Three-part fix:

1. udisksstoragedrive.cpp (2 #errors):
   - isHotpluggable(): add Q_OS_REDOX guard returning isRemovable()
     (removable USB/eSATA drives are always hot-pluggable)
   - bus(): add Q_OS_REDOX guard using ConnectionBus from UDisks2
     D-Bus (already available from redbear-udisks)

2. udisksopticaldisc.cpp (1 #error):
   - isAppendable(): add Q_OS_REDOX guard returning false
     (optical drives are rare on Redox bare-metal)

3. kf6-solid/recipe.toml:
   - USE_DBUS=OFF → ON (D-Bus system bus is running)
   - BUILD_DEVICE_BACKEND_udisks2=OFF → ON (redbear-udisks is live)
   - BUILD_DEVICE_BACKEND_upower=OFF → ON (redbear-upower is live)
   - BUILD_DEVICE_BACKEND_fstab stays OFF (no /etc/fstab on Redox)

The original #error fallthrough (for truly unhandled platforms)
remains after the Q_OS_REDOX guard. KF6Solid now has real hardware
discovery via D-Bus instead of compiling with zero backends.

local/recipes/kde/kf6-solid/recipe.toml

@@ -44,10 +44,10 @@ cmake "${COOKBOOK_SOURCE}" \
     -DWITH_NEW_SOLID_JOB=OFF \
     -DWITH_NEW_POWER_ASYNC=OFF \
     -DWITH_NEW_POWER_ASYNC_FREEDESKTOP=OFF \
-    -DUSE_DBUS=OFF \
+    -DUSE_DBUS=ON \
     -DUDEV_DISABLED=ON \
-    -DBUILD_DEVICE_BACKEND_udisks2=OFF \
-    -DBUILD_DEVICE_BACKEND_upower=OFF \
+    -DBUILD_DEVICE_BACKEND_udisks2=ON \
+    -DBUILD_DEVICE_BACKEND_upower=ON \
     -DBUILD_DEVICE_BACKEND_fstab=OFF \
     -Wno-dev
 

local/recipes/kde/kf6-solid/source/src/solid/devices/backends/udisks2/udisksopticaldisc.cpp

@@ -371,6 +371,9 @@ bool OpticalDisc::isAppendable() const
     return m_udevDevice.deviceProperty(QStringLiteral("ID_CDROM_MEDIA_STATE")).toString() == QLatin1String("appendable");
 #elif defined(Q_OS_FREEBSD) || defined(Q_OS_OPENBSD)
     return m_device->prop(QStringLiteral("bsdisks_IsAppendable")).toBool();
+#elif defined(Q_OS_REDOX)
+    // Redox rarely has optical drives; default to non-appendable.
+    return false;
 #else
 #error Implement this or stub this out for your platform
 #endif

local/recipes/kde/kf6-solid/source/src/solid/devices/backends/udisks2/udisksstoragedrive.cpp

@@ -42,6 +42,10 @@ bool StorageDrive::isHotpluggable() const
     /* clang-format on */
 #elif defined(Q_OS_FREEBSD) || defined(Q_OS_OPENBSD)
     return m_device->prop(QStringLiteral("bsdisks_IsHotpluggable")).toBool();
+#elif defined(Q_OS_REDOX)
+    // On Redox, removable media (USB, eSATA) are always hot-pluggable.
+    // UDisks2 exports the Removable property for all removable block devices.
+    return isRemovable();
 #else
 #error Implement this or stub this out for your platform
 #endif
@@ -96,6 +100,9 @@ Solid::StorageDrive::Bus StorageDrive::bus() const
         m_udevDevice.deviceProperty(QStringLiteral("ID_BUS")).toString();
 #elif defined(Q_OS_FREEBSD) || defined(Q_OS_OPENBSD)
         m_device->prop(QStringLiteral("bsdisks_ConnectionBus")).toString();
+#elif defined(Q_OS_REDOX)
+        // Redox uses UDisks2 D-Bus; ConnectionBus is available from the device.
+        bus;
 #else
 #error Implement this or stub this out for your platform
 #endif