fix: play works on fresh Steam accounts and on Windows

[edersonff]

• Play on a Steam account that never had a non-Steam game now creates shortcuts.vdf instead of failing with "steam shortcuts.vdf not found".
• The game is registered in the signed-in account's shortcuts, not whichever userdata folder is listed first.
• Steam is found through steamlocate: Windows registry, Flatpak, Snap and debian installs; Flatpak Steam is started with flatpak run.
• Windows shortcuts no longer carry Wine DLL overrides, and Play no longer restarts Steam to write a Proton mapping Windows never uses.
• Background checks (tasklist, reg) no longer flash console windows on Windows.
• Game folders with : ? * | < > or reserved names install on Windows; existing Linux folders keep their old names.
• Plugin DLLs from RAR archives land in BepInEx/plugins on Windows too.
• Launch failures show one sentence telling the player what to do; the raw cause goes to the app log.
• CI runs core and app tests on windows-latest and ubuntu-latest.

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