[UX] Show Flatpak's runtime version when missing mangohud/gamescope

[arielj]

When using Heroic as a Flatpak and mangohud or gamescope are not found, the message only told the user to install them and add them to the PATH, when the solution is to actually install the flatpak packages of those tools (because users might see they actually have the tools installed and in the PATH, but they might not know they need the flatpak version)

Since they also have to install the correct version matching the Flatpak runtime, I included that in the message to make it more obvious.

This should solve many issues of users not knowing how to solve the missing mangohud/gamescope problems.

Also, since isFlatpak is a static value that can never change, we don't need an api endpoint, we can just inject the value into the window object at boot using the preload script (we are already sending some static data there), this removes a little bit of complexity in a few places, which I included in this PR.

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Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)

[flavioislima]

lgtm