Feature request: Make inline image display height configurable (in rows)

[zipproth]

Request

Add a config option to control the height at which inline images are displayed, measured in terminal rows.

Background

Inline image height is hard-coded in src/ui/content.rs via const INLINE_THUMBNAIL_HEIGHT: u16 = 4;. It governs both:

• the reserved content height — base_height + inline_images.len() * INLINE_THUMBNAIL_HEIGHT, and
• the rendered thumbnail rect — height: INLINE_THUMBNAIL_HEIGHT, advanced per image.

On a large preview pane this leaves images only ~4 rows tall with most of the pane unused. (A separate cursor-focused image render exists in the same file; ideally it honors the same setting.)

Suggested shape

A Config field, e.g. image_height: u16 (rows), defaulting to 4 so existing behavior is unchanged unless set — fitting the existing #[serde(default = "...")] pattern in src/config.rs. The INLINE_THUMBNAIL_HEIGHT use-sites would read it instead of the constant.