♻️(frontend) major chat refactoring

- Dissociate the chat observer (which persists messages in a store)
  from chat rendering.
- Do not keep the chat mounted in the DOM anymore.
- Recompute the minimal amount of data when a participant renames.
- Memoize most of the layout.
- Drop the manual textarea row-size computation: recent React Aria
  already handles it.

Known regressions still to solve: focus behavior on the input,
scroll behavior on message updates, and edge cases around list
sizing.

Not sure yet whether there is a better way to memoize the
virtualized list; open to feedback.

src/frontend/src/features/chat/components/ChatSubmitButton.tsx

@@ -0,0 +1,30 @@
+import React from 'react'
+import { useTranslation } from 'react-i18next'
+import { Button } from '@/primitives'
+import { RiSendPlane2Fill } from '@remixicon/react'
+
+type ChatSubmitButtonProps = {
+  handleSubmit: () => Promise<void>
+  isDisabled: boolean
+}
+
+export const ChatSubmitButton = React.memo(
+  ({ handleSubmit, isDisabled }: ChatSubmitButtonProps) => {
+    const { t } = useTranslation('rooms', { keyPrefix: 'controls.chat.input' })
+    return (
+      <Button
+        square
+        invisible
+        variant="tertiaryText"
+        size="sm"
+        onPress={handleSubmit}
+        isDisabled={isDisabled}
+        aria-label={t('button.label')}
+      >
+        <RiSendPlane2Fill />
+      </Button>
+    )
+  }
+)
+
+ChatSubmitButton.displayName = 'ChatSubmitButton'