Simple skip: manual offsets and chapter-marked breaks

The last of Phase 2's three tracks, and the part Peter asked for in place of the
audio fingerprinting, which stays deferred.

Two tiers. Manual per-show offsets start each fresh episode past the intro and
treat it as finished before the outro; resuming something part-heard still lands
where it was left, and an offset longer than the episode is dropped rather than
obeyed, because someone typing minutes where seconds were meant should not have
a show start at the end. The fields have been on SubscriptionEntity since phase
1 with nothing reading them.

Chapters are the exact tier: where a show publishes a Podcasting 2.0 chapter
file, the player lists it and taps seek. Breaks the publisher marked `toc:
false` can be skipped, opt-in per show. Nothing is guessed from chapter titles
-- a show discussing its own sponsorship deal has a chapter called "Sponsors"
that is the actual programme, and a skip feature that eats an episode is worse
than none. A break whose end cannot be worked out plays rather than throwing the
playhead somewhere arbitrary.

Two real bugs surfaced while verifying this on a device, both older than this
work:

A start position handed to `seekTo` after `prepare()` is silently dropped across
the MediaController-to-service boundary. The intro offset began at zero, and so,
all along, did every resume -- an episode left half-heard restarted from the
top. Nothing failed and nothing logged. It now goes to `setMediaItem`, verified
on a device both ways: a fresh episode starts at the offset, a stored position
resumes at 150s and plays on from there.

Finishing an episode did not stop playback when the queue was empty, because
until now the file had always ended anyway. With an outro offset the episode
finishes while audio is still playing, so it marked the episode played and then
played the credits regardless.

Schema v3 -> v4: the chapter file cached per episode, and the per-show opt-in.
The migration test now covers 1 -> 4 straight through and asserts the opt-in
arrives off -- defaulting it on would have started removing audio from every
show at once.

Per-show settings moved to their own view model. Drip, the archive fetch and
both skip settings are one concern used by one screen, and ListenViewModel had
become the entry point for the library, the queue, episode actions and these as
well. It also gets its own busy flag: fetching an archive and refreshing a feed
are different operations, and one spinner for both had the top bar claiming a
refresh was running when it was not.

Verified on a device against a real feed throughout: a 60-second intro offset
starts the episode at 60s, an outro offset finishes it early and stops the
audio, and a seeded chapter file with a five-second marked break plays 0-5s then
jumps to 45s with the chapter list rendering alongside.

No permission changes. 364 unit tests green, detekt and lint clean, coverage
65.30 -> 66.23.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018AwnALZL6bD4VBtATDVv3B

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