fix(herdr): make workspace bar background transparent

[mdj2812]

Summary

• Set panel_bg to reset in the Herdr template so the workspace/tab bar uses the host terminal background instead of a solid surface color
• Matches the existing sidebar_bg = "reset" behavior for transparent panel chrome

Test plan

• Enable the Herdr template in Noctalia with an existing ~/.config/herdr/config.toml
• Apply a theme change and confirm [theme.custom] has panel_bg = "reset"
• Verify the workspace bar background is transparent after herdr server reload-config

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