feat(tui-big-text): add optional Block wrapping for BigText

[ololonly]

Summary

Add optional block wrapping for the big text widget so it can render within a border and title area.

Motivation

This enables composing the big text widget with standard block decorations, which is useful for dashboards and status displays.
E. g. I'm making layout with BigText stopwatch and I want to use block title as a place for status. Additionally it fits into my dashboard design.
I think it might be useful.

My example:

Changes

• Added optional block support in the widget configuration and rendering flow.
• Added a test to validate rendering with a bordered block and title.

Testing

cargo test -p tui-big-text --all-features -- --nocapture