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

• I think I picked the correct source and target branch.
• I included a new entry to the CHANGELOG.md.
• 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@             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!