feat(settings): add option to disable existing file check (#848)

[SnitchTeam]

Summary

Adds a third value (none) to the existingFileCheckMode setting, exposed in the Settings UI as Disabled. When selected, the app no longer skips a track because a file with the same name already exists on disk, so the same track can be downloaded into multiple playlist folders.

Behavior

• filename (default) and isrc modes are unchanged.
• none short-circuits ResolveOutputPathForDownload to always return (path, false), bypasses the existing-file early returns in the Qobuz, Tidal and Amazon download paths, and disables the _NN suffix renaming.
• Configs that set the value to off or disabled are normalized to none for forward-compatibility.

Test notes

• go build ./... and go vet ./backend/... pass.
• Manual: with the option set to Disabled, downloading a playlist that contains a track already on disk re-downloads the file instead of skipping it. With Filename or ISRC the previous behavior is preserved.

Closes #848