Migrate to PySide6 with full feature set, theming, and CI/CD pipeline

[Imran-S-heikh]

Summary

Complete rewrite of Proteus from customtkinter to PySide6 with a modular
architecture, new features, and a production-ready packaging pipeline.

UI

• Migrated from customtkinter to PySide6 (Qt) for native look and full styling support
• Modular layout: TopBar + Sidebar + Canvas + StatusBar
• Light / Dark / High-Contrast themes with one-click toggle, persisted across sessions
• Collapsible sidebar sections, zoom controls in status bar

Features

• 4 Sharpen options: Original (unsharp mask), B/W Auto (Otsu), B/W 128, B/W Custom
• PCA navigation: Prev Result / Next Result with Component N of M status
• Pseudocolor merge with custom band labels (e.g. UV + IR)

Packaging

• PyInstaller spec optimized to ~145MB (down from 800MB+)
• One-command build scripts for Linux, macOS, and Windows
• Prebuilt icons (.ico + .icns) committed to repo

CI/CD

• GitHub Actions CI: tests on Ubuntu, Windows, macOS on every push
• GitHub Actions Release: builds all 3 platforms and creates GitHub Release on tag push