fix: coordinate mismatch in EnsureCursorVisible() causing 4-pixel scroll margin

[Bert0ns]

Issue: in TextEditor::EnsureCursorVisible() where typing near the horizontal edges of the window would cause the view to scroll by exactly 1 character per keystroke, effectively pinning the cursor to the physical edge of the editor view.

The Bug

The previous implementation mixed up character column indices and physical pixel floats when attempting to calculate padding bounds:

1. len (from TextDistanceToLineStart) correctly returned physical pixels (float).
2. left and right were calculated by dividing scroll dimensions by mCharAdvance.x, returning character columns (int).
3. The comparison len + mTextStart < left + 4 thus incorrectly compared pixels directly to character columns.
4. Furthermore, ImGui::SetScrollX() expects a pixel value. By calling ImGui::SetScrollX(... - 4), the logic intended to subtract a margin of 4 characters, but mathematically subtracted exactly 4 physical pixels instead.

The Fix

This PR fully normalizes the EnsureCursorVisible logic to use consistent, floating-point pixel math for all boundary and padding calculations.

• It introduces proper pixel-aware horizontal padding (padX = 4 characters).
• It removes arbitrary vertical padding, allowing the cursor to rest naturally at the top/bottom edges of the window without forcing unnecessary vertical scrolling.
• It fixes a bug where ImGui::Dummy (which defines the maximum horizontal scroll bounds) did not account for padding. By adding the horizontal padding to the Dummy width, it ensures that padding is maintained even when typing on the longest line of the document.
• It bounds the padding dynamically via std::min(..., width * 0.25f) to ensure that if the editor window is resized to be extremely small, the padding doesn't mathematically overlap and cause the scroll position to oscillate.

Testing

• Verified on a standalone integration: Typing continuously towards the right margin now smoothly scrolls the view while maintaining a comfortable, properly calculated 4-character margin ahead of the cursor.
• Verified that horizontal padding is correctly applied even on the absolute longest line of the document.
• Ensured no new ImGui internals are called and no memory is allocated.

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