Replace SASS variable-based theming with CSS custom properties for
easier runtime customization. This allows users to override theme
colors without needing a SASS build pipeline.

- Add CSS custom properties for all theme colors with light defaults
- Replace ace-diff-dark.scss with ace-diff-twilight.scss preset
- Update build to compile twilight theme as separate CSS file
- Add theming documentation to README
- Add Cypress tests for theme CSS verification
- Update TEST.md with improved testing documentation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Implements the lockScrolling option (now enabled by default) that
synchronizes scrolling between left and right editors. Uses proportional
positioning to handle documents of different sizes - scrolling to 50%
in one editor scrolls the other to 50% as well.

Closes #21

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Adds a callback that is invoked after diffs are computed and displayed.
The callback receives the diffs array, allowing users to scroll to the
first diff or implement diff navigation.

Closes #57

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Use Cypress retry mechanism instead of fixed wait for more reliable
scroll sync assertion.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

- Wait for Ace gutter cells before testing scroll sync
- Force Ace editor resize in headless mode to ensure lineHeight is calculated
- Add defensive lineHeight fallback for headless environments
- Upload Cypress screenshots/videos as artifacts when tests fail

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Features from PR #94 cleaned up for current codebase:

- Add `charDiffs` option (default: true) to highlight specific changed
  characters within lines, not just whole lines
- Add gutter decorations to mark changed lines in the editor gutter
- Add `clear()` API method to clear diffs without destroying editors
- Add CSS classes `diffChar` and `diffGutter` for styling
- Clean build script to remove .parcel-cache before building
- Add guard checks in clearGutter/clearArrows for robustness
- Remove dark theme import from main bundle (use separately)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Add character-level diff highlighting and gutter decorations

Disable code folding

Add scroll locking feature with proportional sync

Add onDiffReady callback option

Refactor theming to use CSS custom properties