testfeat: Add initial pronunciation practice tool

[jokemon]

This commit introduces a new web application designed to help you improve your English pronunciation.

Key features include:

• File Upload: You can upload a TXT file containing English text and an MP3 audio file of your reading.
• Speech-to-Text (STT): The backend uses Google Cloud Speech-to-Text (GCP STT) to transcribe your audio. (Requires GCP setup).
• Text Comparison: The original text is compared with the transcribed text using difflib to identify discrepancies.
• Results Display: The frontend shows the original text, the transcribed text, and a highlighted diff of the comparison.
• Issue Storage: Identified pronunciation differences (problematic words/phrases) are stored in your session.
• Exercise Page: A dedicated page (/exercise) displays these stored issues, providing targeted practice material.
• Backend & Frontend: Implemented using Flask for the backend and vanilla JavaScript for the frontend.
• Unit Tests: Basic unit tests for the Flask backend logic have been added, covering file handling, STT mocking, session management, and page rendering.

The tool is structured within the pronunciation_practice_tool directory, containing templates, static assets, upload handling, and tests.