refactor: Split src/app.rs monolith into src/ui/ submodules

[Rowrow620]

Description of the Feature

src/app.rs has grown to over 6,900 lines of code in a single file. Splitting this file into a modular src/ui/ directory will reduce merge conflicts for open-source contributors and make the codebase easier to navigate.

Proposed Solution

Refactor UI rendering code from src/app.rs into submodules inside src/ui/:

• src/ui/sidebar.rs: Roadmap navigation and problem search
• src/ui/inspector.rs: Python trace code inspector and scope variables
• src/ui/dashboard.rs: Full-screen NeetCode 150 Mastery grid
• src/ui/modal.rs: Settings and Keyboard Shortcuts popup windows

Implementation Steps

1. Create directory src/ui/ and define module exports in src/ui/mod.rs.
2. Move sidebar rendering functions from src/app.rs to src/ui/sidebar.rs.
3. Move code trace and scope rendering functions to src/ui/inspector.rs.
4. Move full-screen dashboard grid code to src/ui/dashboard.rs.
5. Update src/app.rs to call submodule render methods.
6. Verify build health by running cargo test and cargo fmt --all -- --check.

Acceptance Criteria

To merge a pull request for this issue:

• Code Formatting: cargo fmt --all -- --check completes without warnings.
• All Tests Pass: cargo test passes 100% of unit tests.
• Architecture: src/app.rs size is significantly reduced and UI components live in src/ui/.
• Clean Git Branch: Changes are committed to a feature branch targeting dev.

Step-by-Step Contribution Guide

1. Claim this issue: Comment below that you are working on this issue to receive assignment.
2. Fork and Branch: Create a new branch off dev named refactor-app-submodules.
3. Make and Test Changes: Run cargo fmt --all and cargo test locally to verify build health.
4. Submit PR: Push your branch to GitHub and create a Pull Request targeting the dev branch.

Additional Information

• Contributing Guide
• README