feat: add estimated reading time for books (#606)

[aarushii25]

📖 feat: Add Estimated Reading Time for Books (Fixes #606)

Hey @devanshi14malhotra! I've implemented the estimated reading time feature as described in Issue #606. Here's a full breakdown:

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📋 What Was Done

Added a frontend-only estimated reading time system that calculates and displays how long a book will take to read, updating dynamically based on reading progress.

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🗂️ Files Changed

New File:

• frontend/js/readingTime.js — Reusable utility with three exported functions:
  • calcReadingTime(pageCount) — total reading time from page count
  • calcRemainingTime(pageCount, progressPercent) — time left based on progress
  • formatReadingTime(minutes) — formats minutes into human-readable label

Modified Files:

• frontend/pages/library.html — Added reading time stat item in book detail modal's book-stats-row
• frontend/js/library-3d.js — Reading time badge on book spine + modal population
• frontend/js/app.js — Reading time badge on book back face cards
• frontend/css/style.css — Styled .reading-time-badge and .reading-time-remaining badges

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✅ Formula Used

words  = pageCount × 250   (avg words per printed page)
time   = words ÷ 250       (avg adult reading speed: 250 WPM)
∴ time (minutes) = pageCount

Pages	Reading Time
150 pages	~2 hrs 30 mins
300 pages	~5 hrs
500 pages	~8 hrs 20 mins

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🎯 Where It Appears

1. Book Detail Modal — "Est. Reading Time" shown in stats row. For books being currently read, shows "X hrs left" based on progress percentage.

2. Book Spine (3D shelf) — Small vertical time badge on the spine face.

3. Book Back Face (Discovery cards) — Badge showing total time or remaining time.

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📱 Behavior

• Shows "~4 hrs" format for full books
• Shows "~35 mins left" for currently-reading books
• Shows "Finished!" at 100% progress
• Gracefully hides if page count data is unavailable
• No backend changes required — fully frontend

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🔗 Related

Closes #606

GSSoC'26 contribution — frontend assigned to @aarushii25

Let me know if any adjustments are needed! 🙏

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