Add private reading analytics and a calendar view

[mokhov]

Problem

Readers cannot see when, how long, or how consistently they read without using a separate tracking service.

Proposed behavior

Record optional local reading sessions and present book-level and aggregate calendar views.

Acceptance criteria

• Analytics distinguish active reading from loading, background tabs, and long idle periods.
• Views include daily time, pages or progress, calendar activity, and trends by book.
• Statistics work without an account or external service.
• Tracking can be disabled and all analytics can be cleared independently.
• The feature has bounded storage and does not affect page-turn performance.