Add default background image for rooms (like WhatsApp and Telegram)

### Your use case

Issue Description:
Currently, the chat rooms have a plain, empty background. Unlike WhatsApp and Telegram, there is no default background image or wallpaper to make the chat interface more visually appealing and less枯燥 (monotonous).

Suggestion / Feature Request:
Please add a default background image / wallpaper for all chat rooms, similar to WhatsApp and Telegram. This could be:

A subtle, light pattern or gradient

An option to choose from 3–5 built-in wallpapers

Later, an option for users to set custom backgrounds

Expected behavior:
When a user opens any room, instead of a solid white/gray background, they see a soft default wallpaper that makes the chat interface feel more familiar and comfortable.

### Have you considered any alternatives?

-

### Additional context

Even a simple default wallpaper (like Telegram's dotted pattern or WhatsApp's subtle texture) would greatly improve the visual experience. This feature is especially requested by users coming from other messaging apps.

### Are you willing to provide a PR?

Yes

The issue names no files or tests, so first locate the room screen and inspect how its current solid background is applied. Add a subtle default wallpaper for chat rooms, then verify that opening rooms shows it consistently without reducing message readability.

Written by the indexing model from the issue text.