fix(slideshow): stop on last slide in linear mode; reshuffle endlessly in shuffle mode

[lstein]

Problem

The autoplay slideshow misbehaved at the end of the slide list:

1. Linear mode — on reaching the last image, the slideshow jumped back ~10 slides instead of stopping. Swiper 11's autoplay, on reaching the end with loop: false, calls slideTo(0) — which lands on the first slide of the windowed in-memory buffer (a subset, not the album's first image), so it appeared to leap backward.
2. Shuffle mode — on a small album it showed every image once and then froze (the play indicator still showed "running" but slides stopped changing). The random picker refused any image already loaded, so once all images were in the buffer nothing could be appended.

Fix

Linear end-of-list — We now own end-of-list behavior instead of relying on Swiper's autoplay. When slideState.resolveOffset(+1) reports there is no next slide (genuine end of list, wrap off), the slideNextTransitionStart handler stops autoplay so the slideshow rests on the last slide. stopOnLastSlide: true is kept as a config backstop for the "start the slideshow while already parked on the last slide" case. With wrap on, resolveOffset(+1) returns the wrapped index, so the next slide is appended ahead and the slideshow loops to the start.

Shuffle reshuffling deck — Replaced the "pick a random index not already loaded" logic with a reshuffling bag: every image is dealt exactly once per cycle in random order; when the bag empties it refills and reshuffles into a fresh order, so shuffle runs indefinitely. A cross-cycle guard prevents an image repeating twice in a row (including avoiding an immediate repeat of the on-screen image when shuffle first starts). trimShuffleBacklog keeps the buffer bounded by the high-water mark — re-dealing images across cycles would otherwise grow the DOM without limit — and does so without stop/starting autoplay (so the play/pause icon doesn't flicker every slide).

Behavior after the fix

• Linear + wrap off → stops on the last slide
• Linear + wrap on → wraps to first and continues
• Shuffle → runs forever: shows all images, reshuffles, repeats in a new order

Testing

• Reworked the shuffle unit tests for the new bag semantics (deals every image once per cycle then reshuffles with no boundary repeat; appends a fresh slide even when all images are already loaded — the no-stall regression).
• Added end-of-list coverage: linear stop at genuine end (wrap off), wrapped-slide append (wrap on), shuffle never stops at the last index.
• Full frontend suite passes (343 tests); npm run lint and npm run format:check clean.
• Verified live in the app across all three modes.

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