fix(theme): fix low-contrast annotation color in default and autumn themes

[hexonal]

Fixes #3539

The default and autumn themes both set Meaning::Annotation to Color::DarkGrey in crates/atuin-client/src/theme.rs. This meaning styles the directory/user/host columns and header in the history TUI popup (history_list.rs, interactive.rs, inspector.rs), and on many dark-background terminals DarkGrey renders as nearly the same color as the background, making that text unreadable.

The marine theme doesn't have this problem because it already uses "steelblue" for Annotation. This PR changes default and autumn to use the same "steelblue" value, which keeps reasonable contrast on both light and dark terminal backgrounds.

Added a test asserting default, autumn, and marine all resolve Annotation to the same color, so a future revert to DarkGrey would fail CI.

[greptile-apps]

Greptile Summary

This PR improves annotation contrast in built-in themes. The main changes are:

• Uses steelblue for annotations in the default and autumn themes.
• Adds a test covering annotation colors in default, autumn, and marine.

Confidence Score: 5/5

This looks safe to merge.

• The color already works in the marine theme.
• The test checks all three themes through normal theme loading.
• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
crates/atuin-client/src/theme.rs	Updates two built-in annotation colors and adds focused tests through the normal theme-loading path.

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