Add persistent reader settings and progress - #34

Summary

• add mobile-first appearance controls for theme, font, text size, line spacing, margins, and content width
• persist stable reading position, chapter progress, completion state, and deterministic whole-book progress locally
• add browser-state contract tests and run them in CI

Verification

• 18 Python tests passed with 94.74% coverage
• 5 browser-state tests passed
• Ruff formatting and linting passed
• mypy passed
• visually checked at 390x844 and 1440x900
• verified theme contrast, reload restoration, cross-chapter progress, and completion persistence

Addresses #8
Addresses #9
Addresses #10