Fix memory leaks in reader components

[gvonness-apolitical]

Summary

Memory leak audit identified and fixed 4 critical issues:

• EPUB reader: Event listeners (keydown, relocate) were never removed
• EPUB reader: Page change callback held references after destroy
• Engine: Callbacks held closure references after destroy
• ManifestController: Chapter cache grew unbounded with no eviction

Changes

EPUB Reader (src/readers/epub-reader.ts)

• Store event handler references as instance variables
• Remove keydown listener from document in destroy()
• Remove relocate listener from view in destroy()
• Nullify onPageChangeCallback in destroy()

Engine (src/core/engine.ts)

• Nullify all callback references (onError, onPageChange, onChapterChange, onReady) in destroy()

ManifestController (src/core/manifest-controller.ts)

• Implement LRU cache with max 5 chapters
• Track chapter access order for proper eviction
• Evict least recently used chapters when cache is full
• Nullify callbacks in destroy()

Test plan

• Build succeeds
• Unit tests pass (32/32)
• E2E tests pass (Chromium - 12/12)