Add Plasma applet package skeleton for openwheel-gadget

[fredaime]

Motivation

• Provide an initial Plasma 6 KPackage skeleton so the gadget can be packaged and loaded as a Plasma applet for validation.
• Bring the CMake configuration in line with KDE/Plasma packaging helpers to enable proper installation via KPackage.

Description

• Updated openwheel-gadget/CMakeLists.txt to require CMake 3.16, find ECM and KF6 (including KPackage and I18n) and call kpackage_install_package(package org.openwheel.gadget).
• Added package/metadata.json with KPackageStructure set to Plasma/Applet and basic plugin metadata for org.openwheel.gadget.
• Added a minimal package/contents/ui/main.qml containing a Kirigami.InlineMessage with i18n-ready text i18n("OpenWheel gadget placeholder") as a placeholder UI.

Testing

• No automated tests or CI build were run for these changes.

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Codex Task