fix(mobile): navigate grouped toc items

[codedogQBY]

Analysis

• Some EPUBs expose grouping entries in the TOC, such as volume/part nodes, without their own href.
• The mobile TOC row handler only selected item.href, so tapping those visible entries silently did nothing even when their descendants were readable chapters.

Changes

• Add getFirstTocHref in core reader utilities to resolve the first usable href in a TOC branch.
• Use that helper in the Expo reader TOC row so grouped entries navigate to their first readable child.
• Keep the chevron as the explicit expand/collapse control; entries with no reachable href still toggle expansion.
• Add focused unit coverage for direct href, descendant href, blank href, and no href cases.

Verification

• pnpm exec biome check packages/core/src/reader/toc.ts packages/core/src/reader/toc.test.ts packages/core/src/reader/index.ts packages/app-expo/src/screens/reader/TOCTreeItem.tsx
• pnpm --filter @readany/core test -- toc
• pnpm --filter @readany/core exec tsc --noEmit
• pnpm --filter @readany/app-expo exec tsc --noEmit
• git diff --check

Fixes #404