fix(syncplay): send full series queue and resume position on initiate

[JenteJan]

Builds on the SyncPlay work in DonutWare#735 — targets the syncplay branch so it folds into that PR once merged.

Problem

Starting playback from Fladder while in a SyncPlay group didn't work for other participants — most visibly the official Jellyfin clients (verified against the Jellyfin web client and the LG webOS app):

• TV episodes never started on the other client. Movies did, which made it look movie-vs-episode specific.
• "Continue Watching" restarted the group from 0:00 instead of the saved resume position.

Root cause

_playSyncPlay sent a single-item queue: setNewQueue(itemIds: [itemModel.id], playingItemPosition: 0, startPositionTicks: 0).

• Captured WebSocket traffic shows the official client always queues the whole series (with PlayingItemPosition at the chosen episode). A lone single-episode queue is not started by those clients; movies are single-item by nature, so they worked and hid the bug.
• startPositionTicks fell back to 0 whenever no explicit startPosition was passed. The local playback path resolves the resume point via model.startDuration(); the SyncPlay path did not.

Fix

Mirror the local playback path in _playSyncPlay:

• Build the queue with the existing collectQueue(itemModel) (full series for an episode/series/season; empty for a movie).
• Resolve Series/Season → next-up episode (same as createPlaybackModel's firstItemToPlay).
• Send the whole series with the correct playingItemPosition; movies fall back to a single item.
• Fall back to the resolved item's saved resume position so Continue Watching resumes the whole group mid-item.

Also adds the matching cross-client regression scenarios to docs/syncplay-implementation.md per the regression checklist.

Testing

Manually verified in a real group, with Fladder as the initiator and official Jellyfin clients as the other participant — the Jellyfin web client and the LG webOS app:

• Before: official clients didn't start episodes initiated from Fladder; movies worked. Continue Watching restarted at 0:00.
• After: official clients start episodes initiated from Fladder; Continue Watching resumes at the saved position for all participants. Movies unchanged.