refactor(ui): extract close button construction out of TouchUIBindings

[f1cklepickle]

Why

• TouchUIBindings is a bindings class responsible for wiring UI elements to input events. It currently also contains a factory method CreateCloseButton that constructs and styles a button GameObject at runtime.
• Factory/construction logic does not belong in a bindings class. This violates single responsibility and makes TouchUIBindings harder to test and extend.
• Flagged during the code audit of feat/android-touch-controls.

Proposed Change

• Extract CreateCloseButton into a dedicated CloseButtonFactory (or similar small utility) responsible solely for constructing the button GameObject
• TouchUIBindings calls the factory rather than building the button inline
• No behaviour change — this is a pure structural refactor

Acceptance Criteria

• TouchUIBindings contains no GameObject construction or styling logic
• Close button construction lives in its own class with a clear, single responsibility
• TouchUIBindingsTests passes unchanged
• Upgrade panel close button still appears and functions correctly in MainPrototype

Testing / Validation

• Manual:
  • Open MainPrototype, open upgrade panel, confirm close button renders and dismisses the panel
• Automated:
  • EditMode: TouchUIBindingsTests (must stay green)
  • PlayMode: N/A

Context (Optional)

• Related: refactor(player): split PlayerController — extract repair sensor and weapon swap handler #143 (PlayerController refactor — similar single-responsibility cleanup pass)
• Flagged during audit of feat/android-touch-controls
• Milestone: P6 — Prototype Lock & Polish