feature: configurable height/size for individual announcement banners

**Description**
Currently, all announcement banners share the same fixed styling. There's no way to differentiate between banners that serve different purposes, for example, a compact classification label vs. a more prominent maintenance announcement.

**Use Case**
A customer uses a "Classification" banner alongside other announcement banners. The classification banner should be compact/minimal, while announcement banners may need to be larger to draw attention. Today there's no way to control this per-banner.

**Proposal**
Add a configurable height or size option (e.g., compact, default, large) per announcement banner in the appearance settings. This would allow admins to tailor each banner's visual prominence to its purpose.