Use full terminal width in auto-detected wrap mode

[RubMaker]

Summary

This PR removes the 120-column clamp from Glow's auto-detected CLI wrap width path.

Why

Issue #942 requests behavior equivalent to automatically using terminal width (like -w $COLUMNS) without requiring shell-specific variables or external commands.

Glow already had terminal width auto-detection, but the detected value was capped at 120 columns, which caused premature wrapping on wide terminals.

What changed

1. Removed the 120-column cap in auto-detected width resolution.
2. Refactored width resolution into a small helper (resolveWidth) for clearer logic and testability.
3. Added table-driven tests for width resolution behavior.
4. Clarified README wording for default wrap behavior when -w is not provided.

Behavior

• Explicit -w/--width still has highest priority.
• -w 0 behavior is unchanged.
• Non-TTY / detection-failure fallback remains unchanged.
• No changes to long-word/hard-wrap behavior.
• No TUI layout behavior changes.

Tests

• Added unit tests in main_width_test.go covering:
  • explicit width
  • explicit zero width
  • auto-detected width (including >120)
  • detection fallback
  • non-TTY fallback
• Ran:
  • go test -count=1 ./...

Closes

Closes #942