editor: Keep deleted git-gutter marker visible at small custom widths

[somtri]

Objective

• Fixes Git Gutter width does not scale icons properly #62923.
• When gutter.git_gutter_width is set to a small Custom value (e.g. {"custom": 6.0}), the deleted-line marker becomes disproportionately small. It is nearly invisible in the reporter's screenshots, while added/modified markers scale acceptably.

Solution

• The deleted marker's Default arm uses a 0.35 * line_height base; the strip's Default arm uses 0.275 * line_height. That 27% larger base is what keeps the pill-shaped deleted marker visible next to the rectangular added/modified strips.
• The Custom(N) arm previously used N for both branches, dropping the boost. This PR routes the deleted-empty case through a new deleted_marker_base_width(setting, line_height) helper that applies a single DELETED_MARKER_WIDTH_RATIO = 0.35 / 0.275 constant to both arms.
• Default-mode pixels are byte-identical because floor(0.275 * lh * 0.35/0.275) reduces to floor(0.35 * lh). Custom-mode gains the same 1.273× multiplier the default has always had.

Testing

• Added a pure #[test] fn test_deleted_marker_base_width covering three invariants:
  • Default at line_height = 22.0 returns 7px, identical to the previous default arm.
  • Custom(6.0) at line_height = 22.0 returns a width greater than 6.0 (visibility preserved).
  • Custom(0.0) returns 0.0, so the width-0 render path is unchanged.
• Verified locally on Windows: cargo fmt --all -- --check, cargo check -p editor, and cargo clippy -p editor --all-targets -- -D warnings all exit 0.
• I have not visually verified the rendered marker on macOS or Linux. A reviewer with the reporter's setup (git_gutter_width: {"custom": 6.0} on macOS) can confirm the pill stays visible.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards (UX/UI and icon guidelines)
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

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Release Notes:

• Fixed the deleted git-gutter marker becoming nearly invisible when git_gutter_width is set to a small custom pixel value.