feat(text-editor): add configurable leadingDistribution to TextEditorStyle

[faizahmaddae]

Problem

The text editor hardcodes TextLeadingDistribution.proportional in three places across RoundedBackgroundTextField and RoundedBackgroundText. With proportional distribution, extra leading is split according to the ascent:descent ratio (~75%/25% for Latin fonts), causing glyphs to sit in the lower portion of their rounded background rects when lineHeight > 1.0.

Visual effect

Line height	Glyph offset from bg center
1.0	0 px (no extra leading)
1.5	~3 px below center
2.0	~6 px below center
2.5	~9 px below center
3.0	~12 px below center

This makes the text appear visually misaligned inside its background at any non-default line height.

Root cause

TextLeadingDistribution.proportional distributes extra leading proportionally to ascent and descent. Since ascent >> descent for most fonts, the majority of extra space goes above the glyph, pushing it downward within the line bounds. This is the Flutter default for backward compatibility, but TextLeadingDistribution.even (50/50 split) produces visually centered results.

The value was hardcoded in:

1. RoundedBackgroundTextField._buildBackgroundText() — background paint
2. RoundedBackgroundTextField._buildEditableText() — editable text input
3. RoundedBackgroundText constructors — standalone rendering for saved layers

Solution

Add a configurable leadingDistribution property to TextEditorStyle (default: TextLeadingDistribution.proportional for full backward compatibility). All three hardcoded sites now read from the config.

Changes

File	Change
TextEditorStyle	New leadingDistribution property with copyWith support
RoundedBackgroundTextField	Both builders read widget.configs.style.leadingDistribution
RoundedBackgroundText	New leadingDistribution parameter on both constructors, wired to parent TextSpan style and debugFillProperties
LayerWidgetTextItem	Passes config value to RoundedBackgroundText for consistent saved-layer rendering

Usage

// In ProImageEditorConfigs:
textEditorConfigs: TextEditorConfigs(
  style: TextEditorStyle(
    leadingDistribution: TextLeadingDistribution.even, // vertically centered
  ),
),

Backward compatibility

• Default is TextLeadingDistribution.proportional — zero behavior change for existing users
• The property is optional and follows the same pattern as other TextEditorStyle configs
• All existing tests pass without modification