Add audible & visual bell support

[ranisalt]

This pull request adds support for a visual bell feature in the terminal, allowing the screen to briefly flash when a bell event occurs (if enabled in configuration).

• Added a new Bell configuration struct to Config, with a default value and deserialization support, allowing users to enable or disable the visual bell (rio-backend/src/config/mod.rs). [1] [2] [3] [4]

• In the frontend application event handler, added logic to check the bell configuration and trigger the visual bell effect, mark content as dirty, force immediate render, and schedule a redraw to clear the bell after a set duration (frontends/rioterm/src/application.rs, frontends/rioterm/src/constants.rs). [1] [2]

• Updated the Sugarloaf UI library to support rendering a single overlay quad for the visual bell, including state management and render pass logic to ensure the overlay appears above all other content (sugarloaf/src/components/quad/mod.rs, sugarloaf/src/sugarloaf.rs, sugarloaf/src/sugarloaf/state.rs). [1] [2] [3] [4] [5] [6] [7]

I plan to work on audible bell in a future PR, at least for Linux, adding new configuration options to the [bell] section.

[ranisalt]

It is working on my Linux and my Mac machines, but I don't have Windows to test