feat(code-editor): add collapse unchanged lines extension with expand…

[CaioDGallo]

Problem

The built-in collapseUnchanged option from @codemirror/merge doesn't support gradual expand controls (expand up/down N lines). Users viewing diffs with large unchanged regions had no way to incrementally reveal context; they could only see the collapsed placeholder or load full ranges of unchanged lines between diffs.

Closes #1341

Changes

• Added a custom CodeMirror extension that replaces the built-in collapseUnchanged with an expand-enabled collapse widget. Unchanged regions are collapsed with up/down arrow buttons to expand 20 lines at a time, plus a "N unchanged lines" label that expands all. Expand actions are synced across both sides of split-merge views via mergeViewSiblings.
• Removed the built-in collapseUnchanged config and wired the new extension into both split and unified merge views. The extension is skipped when loadFullFiles is true.
• Replaced the old .cm-collapsedLines gradient styles with new .cm-collapsed-context, .cm-collapsed-gutter, .cm-collapsed-expand-btn, and .cm-collapsed-label styles supporting light/dark themes with hover states.
• Added unit tests covering mapPosBetweenSides, buildDecorations, applyExpandEffect (expand up/down/all), and computeInitialRanges (mocked via buildDecorations assertions).

How did you test this?

• Unit tests for all pure logic functions
• Manual testing in the code editor: verified collapse/expand behavior in split and unified diff views, light and dark themes, and with loadFullFiles toggled

Feature Examples

Before

After