ui: split debug logic into DebugGuiApplication

[deanlee]

Extracts all debug-only functionality into a new DebugGuiApplication subclass and keeps GuiApplication focused on production behavior.

Key changes:

• Move debug-only features (scaling, render texture, burn-in shader, touch overlay, grid, profiling) into a new DebugGuiApplication subclass.
• Keep GuiApplication minimal and focused on release behavior (scale 1.0, no debug overlays).
• Add a lazy factory in application.py that instantiates DebugGuiApplication for debug and GuiApplication otherwise.

Benefits:

• Lower runtime overhead in production (no unnecessary debug checks or rendering).
• Cleaner architecture and easier long-term maintenance.

[github-actions]

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