[Design] Button consistency audit

There is a need to ensure consistency in the design of buttons across the OpenEdX mobile platform. Currently, there are variations in button styles and sizes, which can lead to a confusing user experience. This ticket aims to address this issue by implementing a standardized button design system that can be applied consistently throughout the platform. Here are the general heuristics for these updates:

1. Full width (not variable width) buttons by default (button width = screen width - margins)
2. Side by side buttons in step sequences, with their directional icons on the side that matches the direction they point in
3. Stacked buttons by default for options/choices
4. By default, non-directional icons should be placed immediately to the left of text inside buttons
5. Text is centered within buttons (except certain limited exceptions, e.g. social auth buttons have left-aligned text)
6. Exceptions exist with external dependencies/branding requirements

No files, tests, or entry points are named. Start by auditing the existing button implementations across the iOS app against the listed heuristics and identifying the affected screens. Done means the platform applies a consistent button design, while documented branding and external-dependency exceptions remain supported.