replaced `ThreadPicker` component with `ThreadPickerModal` component.
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Depends on D5317 D5239 D5289 D5230
Linear Issue: [[ https://linear.app/comm/issue/ENG-1848/implement-modal-design | ENG-1848 ]]
Design: [[ https://linear.app/comm/issue/ENG-1846/design-modal-ui-on-figma | ENG-1778 ]]