Fix project manager showing "(DEBUG)" in title instead of "Redot Engine - Project Manager"

[MichaelFisher1997]

Fixes #1037

The project manager was incorrectly showing "(DEBUG)" suffix in its window title during debug builds,
instead of showing the proper "Redot Engine - Project Manager" title.

This change modifies the debug title logic in main/main.cpp to only append "(DEBUG)" when not running the
project manager, using Engine::get_singleton()->is_project_manager_hint() to detect this case.

Changes:

• Added condition check to exclude project manager from debug title suffix
• Project manager now displays proper title even in debug builds
• Regular projects still show "(DEBUG)" suffix as intended

Testing:

• Verified project manager shows correct title in debug builds
• Confirmed regular projects still show "(DEBUG)" suffix appropriately

(tested on Debian 13)