refactor: centralize example registry and switching logic (Issue #8)

[amritamishra01]

Description

This PR refactors how preinstalled examples are managed by introducing a structured example registry and centralizing example switching logic.

Changes:

• Added CATEGORIZED_PROGRAMS and ALL_PROGRAMS in utils/preinstalled_programs.js.
• Updated TextBox to render the Examples dropdown from the structured registry instead of iterating directly over preinstalled_programs.
• Centralized example switching logic in pages/index.js via handleExampleChange.
• Updated the default example loading to use the new registry structure.

Purpose:

This improves separation of concerns between data and UI, centralizes selection logic, and makes the examples menu easier to maintain and extend in the future.
This change improves separation of concerns and maintainability without modifying compilation, execution, or existing functionality.

Closes #8