Make shimmer gradient theme-aware and semi-transparent in dark mode

[sinansays]

Motivation

• Dark-mode shimmer was rendering as an opaque bright rectangle instead of a subtle highlight on text lines.
• The shimmer needed to be alpha-based (semi-transparent) rather than a solid white block.
• The effect should automatically adapt to both light and dark themes using shared settings.
• Reuse theme configuration so shimmer intensity scales consistently across themes.

Description

• Extended the theme object in thinking-gif/lib.js to include a shimmer config with r,g,b,minAlpha,maxAlpha per theme.
• Replaced fixed white gradient stops with an rgba(...) gradient derived from theme.shimmer values.
• Compute shimmerAlpha from shimmerIntensity and the theme alpha range to produce a subtle highlight in dark mode.
• Change is localized to thinking-gif/lib.js and preserves the existing per-line clipping and source-atop composite logic.

Testing

• No automated tests were run for this change.

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