[Web/PWA] Add reading statistics and history

[AlphaMao1]

Parent

#11

What to build

Add the personal reading statistics surface from the existing product: reading sessions, active days, total duration, heatmap/history, and per-book reading activity. This should be useful but should not block the core read/upload/AI loop.

Acceptance criteria

• Reading session data is recorded from real reader activity, not static demo data.
• Users can see total sessions, total reading time, active days, and average session information.
• Users can see a heatmap or equivalent history visualization for recent reading activity.
• Statistics sync or restore consistently for a signed-in user where the underlying session data is synced.
• Offline reading sessions are queued locally and reconcile when network returns.
• Empty and insufficient-data states are clear.
• Desktop and Android/narrow layouts follow the approved visual system.

Blocked by

• [Web/PWA] Build EPUB reader core with progress sync and offline read #15
• [Web/PWA][UI] Apply selected visual system to app shell and core screens #23