Fix chapter updates for fast EPUB slider seeks

[AlexAndBear]

Description

Fixes chapter selection desync in the EPUB reader when users drag the reading progress slider quickly (especially to the left).

Changes:

• use relocated event payload as the primary location source in App.vue
• improve chapter resolution to match navigation items by id as well as href
• make spine-order chapter resolution deterministic when direct TOC matches are unavailable
• add unit tests for stale relocated/currentLocation mismatch and ambiguous/frontmatter chapter resolution

Related Issue

• Fixes <issue_link>

How Has This Been Tested?

• test environment: local dev environment (macOS), Vitest
• test case 1: pnpm test:unit --run packages/web-app-epub-reader/tests/unit/app.spec.ts
• test case 2: pnpm test:unit --run packages/web-app-epub-reader/tests/unit/helpers/chapterResolving.spec.ts
• test case 3: fast slider seek scenario covered by new unit tests in app.spec.ts and chapterResolving.spec.ts

Types of changes

• Bugfix
• Enhancement (a change that doesn't break existing code or deployments)
• Breaking change (a modification that affects current functionality)
• Technical debt (addressing code that needs refactoring or improvements)
• Tests (adding or improving tests)
• Documentation (updates or additions to documentation)
• Maintenance (like dependency updates or tooling adjustments)