feat: Improve performance with QThread workers for download and file processing

[Pravin-raj-03]

Closes #29

Summary

This PR introduces two new QThread worker classes (DownloadWorker and RemovalWorker) that move long-running blocking operations off the main UI thread, preventing the GUI from freezing during:

• Exiftool download / update checks
• • Metadata removal file processing

New Files

• Source Code/GUI_Qt6_Source_Code/workerthread.h - Declares DownloadWorker and RemovalWorker QObject-based worker classes
• • Source Code/GUI_Qt6_Source_Code/workerthread.cpp - Implements both workers

How it works

The QObject worker pattern is used (preferred over subclassing QThread):

• Workers are created with QObject::moveToThread()
• • All cross-thread signals use Qt automatic queued connections for thread-safe UI updates
• • DownloadWorker: Downloads exiftool zip with progress reporting via progress(int) signal
• • RemovalWorker: Iterates files, runs exiftool per-file, emits fileProcessed() and progress() signals