Extract prompt UI and tests into separate modules (vertical-slice refactor)

[enijburg]

Motivation

• src/main.rs had grown too large and mixed runtime UI prompts and test code with core viewer logic, reducing maintainability.
• Introduce clearer vertical slices so interactive prompt behavior and tests live in their own modules and the core runtime file is smaller and easier to navigate.

Description

• Extracted prompt_goto_line and prompt_find from src/main.rs into src/ui/prompt.rs and made them pub(crate) so the main runtime can call them via use ui::prompt::{prompt_find, prompt_goto_line}.
• Added a UI module boundary by creating src/ui/mod.rs and wired main.rs to import the prompt functions from the new UI slice (mod ui;).
• Moved the inline test module out of src/main.rs into src/tests.rs and replaced the in-file test block with mod tests; to separate verification code from runtime code.
• Kept existing runtime logic intact; only prompt logic and test placement/visibility were changed to reduce main.rs size.

Testing

• Ran formatter with cargo fmt, which completed successfully.

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Codex Task