fix: chat visibility race condition and persistent fade timers

[Liparakis]

Problem

1. Command Feedback Race Condition: When a user submits a command (e.g., /tpa in @BobbyTheCatfish 's case), ChatBox.OnChatSubmit invokes the command and then immediately calls HideChatInput(). In the previous code, HideChatInput() explicitly called Hide() on every message, which nuked any new addon feedback before it could be rendered.
2. Fade Timer Resets: The message fade-out timer (7.5s) was stored in a local coroutine variable. Opening and closing the chat would stop and restart the routine, resetting the timer to 0 and preventing messages from ever fading if the UI was toggled.

Solution

• ChatBox.cs: Replaced the destructive Hide() loop in HideChatInput() with UpdateMessageVisibility(). This allows the UI to handle visibility state naturally, ensuring that newly added messages remain visible when the input area closes.
• ChatMessage.cs: Migrated the fade timer state (_waitElapsed, _fadeElapsed) to class-level fields. The FadeRoutine now correctly pauses when the chat is open and resumes from its exact progress when the chat closes.

[Extremelyd1]

Nice description of the problem and solution, and clean fix!