fix(wallpaper): deduplicate custom wallpaper entries

[istekhar8966]

This fixes an issue where the same custom wallpaper could be added multiple times
to the selection list, even though it was only stored once in the config.

The UI layer did not check for existing paths before inserting new entries,
which caused duplicate items to appear.

This change ensures that custom images are deduplicated based on their path
before being added to the in-memory selection state.

Before:

After:

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[mmstick]

Make sure to rebase your branch. There are some stale commits in here.

[istekhar8966]

Done. Rebased onto upstream master and cleaned up the commit history. Please take another look.
Thanks!

[jacobgkau]

As described, the wallpaper page used to allow adding the same image multiple times, although it only showed up once after the page was left and reloaded. This PR fixes that bug. All other settings on this page are still working as expected.