New component: chat input

[fabianhueske]

New mt-chat-input component: chat composer for normal + AI chat.

• Multi-line auto-growing input with send affordance
• Enter to send / Shift+Enter newline (configurable); empty/disabled handling
• Optional attachments (slot)
• AI: busy/streaming state with stop/cancel; disables submit while generating; optional slash-commands/suggestions
• Slot for leading/trailing actions
• States: default/disabled/busy/error (retry)
• Labelled input; busy state announced