fix(seek-slider): correct cluster seek rebuild and live drag index

[lstein]

Problem

Reproduce: select an image cluster, enter swiper view, run the sequential slideshow until it stops on the last image, then drag the seek-slider back to image #1.

• It landed on the wrong image (e.g. Workflow/fix release workflow #3 instead of Bump version #1) with a corrupted position badge.
• Swiping left then showed the badge as "0" and "-1".
• It could also occur without the slideshow (harder to reproduce — any seek to an image not currently in the buffer).

Root cause

SwiperManager.seekToSlideIndex rebuilds the windowed slide buffer when the target isn't loaded. The old loop stepped the global album index and the search index together (globalIndex + i, searchIndex + i). That assumes search-result order is contiguous in global-album index, which is false for clusters/searches — their members are scattered across the album. As a result it:

• loaded album-adjacent images instead of the adjacent cluster images, and
• with the -2 origin, tagged the prepended slides with out-of-range search indices (-1, -2).

The position badge renders index + 1 (score-display.js), so those mistagged slides displayed as "0" and "-1", and a slide actually showing image #1 could carry a stale search index and read "3".

Fix

• swiper.js — the rebuild now branches on mode: in search/cluster mode it resolves each neighbour's global index through searchToGlobal(searchIndex + i) and clamps to [0, totalCount); album mode keeps the contiguous globalIndex + i stepping. No slide is ever loaded with the wrong picture or an out-of-range index.
• seek-slider.js — a pre-existing live-drag off-by-one, exposed once the release-time display became correct: onSliderInput passed targetIndex + 1 to showCluster/showSearchScore, which already add 1, so the badge read one too high while dragging. Now passes the 0-based targetIndex, matching the album branch and the release-time path.

Tests

• tests/frontend/seek-search-rebuild.test.js — verifies the rebuild loads adjacent cluster images, never tags slides with out-of-range search indices, and lands on image Bump version #1 with the correct badge. Fails on the old code, passes on the fix.
• tests/frontend/seek-slider-live-index.test.js — verifies the live-drag badge uses the 0-based index for cluster and scored-search results. Fails on the old code, passes on the fix.
• Full frontend suite: 349 passed (24 suites); lint + prettier clean.

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