Add diff line background intensity option

[skripalschikov]

Add diff line background intensity option

Motivation

For me the default diff line background is very pale — green/red lines fade into the page tint and the diff is hard to scan, especially on light themes. This PR adds a user-facing toggle so anyone with the same pain can crank the line bg up without affecting users who like the current look.

Summary

Adds a Line Background Intensity setting (Subtle / Normal / Strong) so users can opt into a bolder green/red line background. Defaults to Subtle — existing users see no visual change.

The control appears as a sub-option under Show Diff Background in both the diff-options popover and the Settings dialog; it's only shown when the parent toggle is on.

Approach

• Subtle preserves the @pierre/diffs library default exactly.
• Normal / Strong scale the line bg through Pierre's existing --mix-light / --mix-dark pipeline rather than overriding the final background-color. This keeps Pierre's --diffs-line-bg flow intact so selected / hovered / decorated line states still produce their state-specific visuals.
• Word-emphasis chip is derived from --diffs-computed-diff-line-bg (oklch(from … calc(l ± 0.07) c h)), not from the theme's addition / deletion base color. The chip ends up "one perceptual step deeper than this specific line" at every intensity and across themes — no per-intensity emphasis tuning needed.
• Theme overrides for --diffs-addition-base / --diffs-deletion-base are respected — only the alpha/lightness scales, not the hue.

Bug fix bundled in

Pierre's word-emphasis rule ([data-line-type=…] [data-diff-span] { background-color: var(--diffs-bg-addition-emphasis) }) isn't gated on [data-background], so disabling Show Diff Background used to leave the word-diff chips painted on plain lines. Added a pre:not([data-background]) … { background-color: transparent !important } rule that applies at every intensity (including Subtle), so the toggle now consistently turns both line bgs and chips off.

Drive-bys

• Dropped dead .diff-line-addition / .diff-line-deletion rules in packages/review-editor/index.css — these classes never actually reach Pierre's shadow DOM, so they had no effect and were misleading for anyone tracing where the diff bg lives.
• Inline hunk previews (DiffHunkPreview — used in Code Tour, comment threads) pick up the same intensity setting, so the look stays consistent with the main diff viewer.

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*Changes authored by Claud

Screenshots (GitHub theme)

Subtle:

Normal:

Strong: