Add Sleep Timer Live Activity

[mofman]

Adds a Sleep Timer Live Activity so users can see the remaining sleep timer countdown from the Lock Screen and Dynamic Island.

The Live Activity starts when a timed sleep timer is enabled, updates when the timer is extended, and ends when the timer finishes or is cancelled. It also includes a + 5 Minutes action so users can extend the
timer directly from the Live Activity. The Lock Screen presentation intentionally keeps the timer card focused on the countdown, since episode details are already shown in the system Now Playing card.

To test

1. Build and run the app on an iOS 17+ simulator or device.
2. Start playing an episode.
3. Open the sleep timer and choose a timed duration, for example 30 minutes.
4. Lock the device.
5. Confirm the Sleep Timer Live Activity appears on the Lock Screen with the countdown.
6. Tap + 5 Minutes.
7. Confirm the countdown increases by 5 minutes.
8. Unlock the device and cancel the sleep timer.
9. Lock the device again and confirm the Live Activity is dismissed.
10. Repeat with the Dynamic Island expanded and compact states on a supported simulator/device.

Checklist

• I have considered if this change warrants user-facing release notes and have added them to CHANGELOG.md if necessary.
• I have considered adding unit tests for my changes. (not necessary)
• I have updated (or requested that someone edit) the spreadsheet to reflect any new or changed
  analytics.

No new analytics were added.

[CLAassistant]

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[mofman]

[CookieyedCodes]

Is it possible to add a +1 min option too & a different dynamic island if chapters or episode timers are selected?

[kean]

Hey, thanks for the PR @mofman! I'm sharing it with the product folks to gather some feedback before the review.