fix: handle settings with missing `shared` section

[rycee]

Description

This allows configuration files without a shared field, using the default value if missing.

Before:

$ pueued -vv -c /dev/null
Error: Error while reading configuration.

Caused by:
    Error while reading configuration:
    missing field `shared`

After:

$ pueued -vv -c /dev/null
21:12:26 [INFO] Restoring state
21:12:26 [INFO] Using unix socket at: "/run/user/1000/pueue_rycee.socket"

Checklist

• [x] I think I picked the correct source and target branch.
• [x] I included a new entry to the CHANGELOG.md.
• [x] I checked cargo clippy and cargo fmt. The CI will fail otherwise anyway.
• [ ] (If applicable) I added tests for this feature or adjusted existing tests.
• [ ] (If applicable) I checked if anything in the wiki needs to be changed.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 80.18%. Comparing base (be40e46) to head (c93ab16).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #505 +/- ## ============================================ Coverage 80.18% 80.18% ============================================ Files 77 77 Lines 5673 5673 ============================================ Hits 4549 4549 Misses 1124 1124 ```

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[Nukesor]

Thanks!