Handle Y offsets in EH preview sprites, migrate preview cache to v2, and update UI

[patrick3399]

Motivation

• Preview sprite sheets now include both X and Y offsets in some formats, so the backend and frontend must preserve and render the vertical offset correctly.
• The preview cache format is being bumped to avoid collisions with old serialized values and allow clients to expect the new offsetY field.
• Frontend components and types must be updated so sprite previews render correctly in the gallery detail and reader thumbnail strip.

Description

• Changed preview cache key from eh:previews:{gid} to eh:previews:v2:{gid} in backend/services/cache.py to version the cache.
• Updated preview HTML parsing in backend/services/eh_client.py to extract both X and Y offsets from new and legacy preview formats, and to store previews as url|offsetX|offsetY|width|height (legacy offsetY omitted handled).
• Normalization logic in _parse_detail_html now computes and applies sprite max heights for the new 5-field format.
• Frontend gallery detail page (pwa/src/app/browse/[gid]/[token]/page.tsx) updated PreviewGrid and preview parsing to accept and normalize offsetY and to use both offsets when rendering sprite backgrounds.
• Reader component updates in pwa/src/components/Reader/index.tsx updated ThumbnailStrip to parse optional offsetY, normalize offsets, and include vertical offset in backgroundPosition and backgroundSize calculations.
• Type definitions updated in pwa/src/lib/types.ts and inline comments to document the new preview string format and legacy handling.

Testing

• Ran backend unit tests with pytest and the suite completed successfully.
• Ran frontend type check and build (pnpm install + pnpm build / tsc) and the TypeScript build succeeded without errors.
• Manual smoke render verification performed by exercising gallery detail and reader thumbnail components to confirm sprite offsets render as expected.

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Codex Task