Convert lessons external link icon to icon button

[MihaiCristianCondrea]

Summary

• replace the lessons list "open in new" affordance with a Material icon button and provide accessibility text
• update the lessons adapter to bind the new button and trigger the lesson intent when pressed
• add a dedicated string resource for the button's description

Testing

• ./gradlew test (fails: SDK location not found in container)

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https://chatgpt.com/codex/tasks/task_e_68cdc42d2bdc832da54885736137f368