[Refactor]: Consolidate duplicate color/theme definitions into a single file

[Sandesh282]

Description:

The app's color palette is currently defined across multiple files, creating duplication and inconsistency. There are two separate theming systems, one of which is entirely unused.

Current Behavior:

1. ContentView.swift (lines 3–20) — Contains Color and UIColor extensions defining the actual colors used throughout the app (darkBackground, neonBlue, neonPurple, etc.).
2. Theme.swift — Contains a separate Theme enum with system colors (Color(.systemBackground), Color.blue) that are never referenced anywhere in the codebase.
3. ProblemModels.swift — Contains an unrelated Color(hex:) initializer extension mixed in with model code.

This structure is confusing for new contributors and makes the design system fragile.

Feature Requirements:

• Create a dedicated color/theme file (e.g., CForge/Theme/AppColors.swift) or repurpose Theme.swift.
• Move all Color extensions from ContentView.swift into this file.
• Move the UIColor extensions from ContentView.swift into this file.
• Move the Color(hex:) initializer from ProblemModels.swift into this file.
• Remove the unused Theme enum or update it to reference the actual neon color palette.
• Ensure ContentView.swift only contains the ContentView struct.

Steps to Implement:

1. Create a new file CForge/Theme/AppColors.swift (or rename/repurpose Theme.swift).
2. Move all Color and UIColor extensions from ContentView.swift into the new file.
3. Move the Color(hex:) extension from ProblemModels.swift into the new file.
4. Delete or update the unused Theme enum.
5. Build (Cmd+B) and verify all views still compile — the extensions are global, so no import changes are needed.
6. Run the app to confirm there are no visual regressions.

Expected Outcome:

• All color and theme definitions live in a single, dedicated file.
• ContentView.swift contains only view code.
• ProblemModels.swift contains only model code.
• The app looks and functions identically to before (pure refactor).