fix: merge custom modules correctly

[Times-Z]

Objective

Fixes #353

Merge [[modules.custom]] entries defined across multiple imported TOML files so that all custom modules are available in the bar.

Solution

Updated the TOML configuration merge logic to concatenate arrays under modules.custom across imports.

Other arrays retain the existing overlay behavior, where the later configuration takes precedence.

Added regression tests covering:

• Merging custom modules from multiple imports
• Preserving replacement behavior for non-custom arrays

No visual changes

Test Plan

• cargo fmt --all -- --check
• cargo test -p wayle-config
• cargo test -p wayle-config infrastructure::loading::merging
• cargo clippy --workspace

[Times-Z]

Mmh, CI fails are not related to my changes, do you wan me to fix them ?