Add inline documentation to ISBN conversion algorithms

[Copilot]

Addresses code review feedback requesting documentation for the ISBN conversion logic in books.html.

Changes

Added inline comments explaining:

• ISBN-10 to ISBN-13 conversion: Why we prepend '978' (bookland EAN prefix) and the EAN-13 check digit algorithm (alternating weights 1/3, modulo 10)
• ISBN-13 to ISBN-10 conversion: Modulo 11 algorithm with descending position weights (10→2), special handling for 'X' (represents 10) and remainder edge cases
• Context on standards: ISBN-13 is the modern replacement for ISBN-10; '979' prefix ISBNs have no ISBN-10 equivalent

Example

// Calculate EAN-13 check digit using the standard algorithm:
// Sum all digits, alternating between weight 1 (even positions) and weight 3 (odd positions)
// The check digit is the number needed to make the total sum a multiple of 10
let sum = 0;
for (let i = 0; i < 12; i++) {
    const d = parseInt(core[i], 10);
    sum += (i % 2 === 0) ? d : d * 3;  // Even index = weight 1, odd index = weight 3
}
const check = (10 - (sum % 10)) % 10;

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