🎨 Palette: Disable chat input during offline state

[teerthsharma]

💡 What

Disables the chat input and send button in the ChatInterface component whenever the DSP Downlink is offline (i.e. tunnelStatus !== 'LOCKED').

🎯 Why

Previously, the input remained active during disconnects, leading to ghost interactions where the user could try to send a message to a severed downlink. This UX enhancement ensures the system state is accurately reflected in the user's controls, preventing confusion.

📸 Before/After

• Before: Input accepts text and the button is clickable, but nothing happens if the WASM bus is disconnected.
• After: Input placeholder reads "Downlink offline...", the text input and send button are disabled, cursor changes to not-allowed, and opacity drops by 50%.

♿ Accessibility

Added explicit aria-labels and title tags that dynamically update to explain why the input is disabled (e.g. "Input disabled while offline", "System offline"), improving context for screen reader users and those navigating via keyboard.

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PR created automatically by Jules for task 1595535961094237573 started by @teerthsharma

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