fix(output): fall back when configured mode is unavailable

[floydya]

Summary

• Fall back to the preferred advertised mode when the configured resolution is unavailable.
• Keep custom modes for outputs without advertised fixed modes.
• Add mode-selection tests and document the fallback.

Motivation

Prevent an output from remaining black when a monitor reconnects without advertising the configured mode.

Type of Change

• Bug fix
• Documentation

Related Issue

Closes #153

Testing

• nix develop -c just format
• nix develop -c just test — 53 tests passed
• nix develop -c just check — 135 checks passed
• Targeted clang-tidy passed for all changed C++ files
• Release package built from the local checkout

Manual Coverage

• Tested in a native Umbriel session
• Tested with multiple monitors

Checklist

• This PR is ready for review, or it is marked as Draft.
• This change fits SCOPE.md, or its scope was agreed in an issue or on Discord first.
• I read and followed the relevant guidance in CONTRIBUTING.md.
• I ran just format, or this PR has no C++ changes.
• I ran the relevant build, test, lint, or verification commands, or explained why they were not run.
• I functionally verified compositor behavior where automated checks are insufficient.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I updated docs/ and examples/config.toml, or this PR does not change user-facing configuration or behavior.
• I used canonical names for config keys, IPC actions, paths, and identifiers.

[ItsLemmy]

Thanks, please re-test is all is alright after changes and merge.