First time setup implementation

[altpyrion]

Purpose of this PR:

Adds a two-step First Time Setup popup that runs before the main window when paths aren't configured yet. Only really working on Linux until now.

How to Test:

What Has Been Changed:

New Features/DolphinManagement feature

• IDolphinLocator / IDolphinInstaller abstractions with a DolphinInstallation(DisplayName, LaunchTarget, Found) record.
• LinuxDolphinLocator detects both Flatpak and native installs.
• Registered through AddDolphinManagement() (Linux-only, behind #if LINUX), replacing AddDolphinInstaller() in SetupExtensions.

Notes

• The old Features/DolphinInstaller folder is still present and still referenced by SettingsManager, WhWzSettings.axaml.cs, and the tests.
• Windows and macOS have no IDolphinLocator/IDolphinInstaller implementation
• Subtext of Flatpak installation entry not updating after successful installation

Related Issue Link:

#300

Checklist before merging

• You have created relevant tests

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