Add the watch's own playback service and controller (issue #276)

[mapitman]

Summary

Step 5 of #276 (see #278-281 for steps 1-4). The watch can now actually stream and play an episode from its own local (synced) queue, independent of the phone.

• WearPlaybackUrlResolver always resolves to FeedItem.enclosureUrl — no downloaded-file branch, since phase 1 is streaming-only (regression guard for phase 2/Send downloaded episodes to Wear OS for offline playback (phase 2) #277's eventual file-transfer support).
• WearPlaybackMediaItemFactory builds a Media3 MediaItem from a FeedItem, trimmed from :app's PlaybackMediaItemFactory: no mobile-data gating, per-feed speed, or volume boost, since none of that settings state is part of what WearQueueSyncApplier syncs onto the watch.
• WearPlaybackService (a MediaSessionService) owns the ExoPlayer instance, auto-advances through the local queue on episode completion, and pushes its position-save loop through WearSyncClient the same way :app's PlaybackService does — trimmed from it: no preload manager/timeline lookahead, no Android Auto browse tree.
• WearPlaybackController is a thin MediaController wrapper exposing WearPlaybackUiState, mirroring :app's PlaybackController's shape, trimmed to what the watch UI (step 6, not this step) will need.

Test plan

• ./gradlew assembleDebug testDebugUnitTest lintDebug passes across :app/:core/:wear
• New unit tests: WearPlaybackUrlResolverTest (regression guard — always enclosureUrl, never a downloaded file), WearPlaybackMediaItemFactoryTest (MediaItem construction, artwork fallback, resume position)
• Installed and launched both debug builds — confirmed no crash on either
• WearPlaybackController isn't called from anywhere yet — no UI exists to trigger play() until step 6 replaces the placeholder MainActivity screen, so there's nothing to manually play-test on-device in this PR