fix: center image captions alongside the total counter

[infeng]

Enabling showTotal shifts the image caption left because the floated counter reduces the centered text area. This is visible in both modal and inline viewers (about 21.55 px with the demo counter).

Group the caption and dimensions in a flexible column, with equal space on either side matching the counter width. Long captions and counters wrap without overlapping; a counter-only row retains its height. The public props are unchanged.

Validation:

• npm run verify passed: 42 tests, lint, library/demo/SSR builds, export/hydration/multipart checks, zero-critical audit gates, and package dry run.
• Chrome 149 geometry checks at 1280/640/360 px and in a 600 px inline container: caption offset changed from -21.55 px to 0 px.
• Checked long caption/localized-length counter at 360 px, showTotal/noImgDetails toggles, navigation counter updates, and an empty caption with a visible counter.

Related to #139. The report remains open for release tracking.