Translate Chapter 7 (Electric Polarization) to English

[Copilot]

Translates Chapter 7 "Polaryzacja elektryczna" from Polish to English, continuing the systematic translation of the book manuscript.

Changes

• Chapter title: "Polaryzacja elektryczna" → "Electric Polarization"
• Section 7.1: "Teoria fazy Berry'ego" → "Berry Phase Theory"
  • Berry phase formalism for periodic systems
  • Relationship between polarization and geometric phase
  • Wannier function representation
• Section 7.2: "Ładunki efektywne" → "Effective Charges"
  • Born effective charge tensor derivation
  • LO-TO splitting in insulators
• Section 7.3: "Ferroelektryki" → "Ferroelectrics"
  • Structural phase transitions in perovskites
  • Spontaneous polarization calculations
  • BaTiO₃ example with figure caption

Preserved

• All LaTeX mathematical formulas and equation labels
• All citations (\cite{berry1,berry2,resta1993,zhong1994,PCM,Zak})
• Figure references and BibTeX keys

Note

Pre-existing issue identified: Line 3267 has incomplete summation index \sum_{=1}^{emp} (missing index variable, likely m). Present in original Polish text.

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[jochym]

@copilot I heve applied the moved the changes from this PR to the different one. Closing this PR since it is redundant.