Allow simpler tab bar appearance customization

[Andiveli]

The tab bar currently has two small customization gaps that make it difficult to keep a minimal, readable layout:

• Active and inactive tab label foregrounds cannot be configured independently without changing broader palette colors that also affect the sidebar and other UI elements.
• The + button remains visible even when new tabs are always created through a keyboard binding.

The first gap matters when a theme default tab contrast is poor. Issue #2692 shows a related readability problem, but changing global muted colors also changes unrelated UI surfaces.

The second gap is mainly about reducing unnecessary visual chrome. Hiding the button should not disable tab creation or change existing keyboard and mouse behavior elsewhere.

A useful outcome would be:

• Separate active and inactive tab foreground customization, compatible with light and dark theme overrides.
• An option to hide only the tab bar + button while preserving every other tab interaction.

These options address the same tab-bar presentation problem and do not change tab identity, navigation, or workspace behavior. Would this level of tab bar customization fit the Herdr UI direction?