fix(ui): initialize button badge colors

[tbracquart]

Summary

Initialize colors for dynamically-created button badges so session shortcut badges are visible immediately.

Motivation

Session shortcut badges displayed their text, but their rounded background was invisible until the button received hover or focus. The badge is created dynamically after the initial button colors are applied, so it did not receive its colors until a later state change.

Type of Change

• Bug fix

Related Issue

Testing

• just format
• just build
• just run
• just test
• Manually verified that the session shortcut badges are visible without hover or focus on Hyprland.

just test completed with 110/112 tests passing. The unrelated noctalia:process and noctalia:time_format tests failed; neither exercises the button badge code changed by this PR.

Manual Coverage

• Tested on Niri
• Tested on Hyprland
• Tested on Sway
• Tested on another compositor:
• Tested with different bar positions and density settings
• Tested at different interface scaling values
• Tested with multiple monitors

Screenshots / Videos

Before

The shortcut badges display their numbers, but their rounded backgrounds are not visible until the button receives hover or focus.

After

The shortcut badges now display their rounded backgrounds immediately, without requiring hover or focus.

Checklist

• This PR is ready for review, or it is marked as Draft.
• I read and followed the relevant guidance in CONTRIBUTING.md.
• I ran just format with clang-format v22+ installed, or this PR has no code changes.
• I ran the relevant build or test commands, or explained why they were not run.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I updated user-facing documentation in docs/user/ when this PR changes documented behavior or configuration, or this PR does not require documentation changes.
• I added or updated assets/translations/en.json, or this PR adds no new user-facing strings.
• I did not edit non-English translation files unless this PR is explicitly for translation tooling, an import/export sync, or a maintainer-requested locale change.
• I used the existing canonical names for config keys, IPC names, paths, and identifiers.