Transcripts: show transcripts through a shelf button

[leandroalonso]

📘 Part of: #1848

This PR:

1. Add a shelf icon so transcripts can be accessed (and remove the tab)
2. Show the transcript in the player view
3. Add a close button to dismiss the transcript

https://github.com/user-attachments/assets/e26e750f-9aeb-47e5-8be1-621e5afe4bc8

To test

Flag disabled

1. With the transcripts flag disabled
2. Open the player
3. Tap "..." on the shelf (below the player buttons)
4. Scroll down
5. ✅ There should be no mention of Transcripts

Flag enabled

1. Enable the transcripts flag
2. Open the player
3. ✅ Transcripts should be the 4th button, if it's not tap "..." and move it to one of the first 4 positions
4. Play an episode from the podcast Cautionary Tales
5. ✅ The transcript button should be enabled
6. Tap it
7. ✅ Transcript should show
8. Swipe down the transcript scrollbar
9. ✅ The whole player should disappear
10. Open the player again and tap the transcript icon
11. Tap the close button ("X" at the top-left corner)
12. ✅ Transcript should disappear

Checklist

• [x] I have considered if this change warrants user-facing release notes and have added them to CHANGELOG.md if necessary.
• [x] I have considered adding unit tests for my changes.
• [x] I have updated (or requested that someone edit) the spreadsheet to reflect any new or changed analytics.