Add configurable spacebar colors per language

[drdanielbender]

Summary

Addresses #1790.

Issue #1790 requests per-language themes as a visual indicator for multilingual use. This implements a focused solution to the underlying problem: each configured language can have its own spacebar color.

Coloring only the spacebar provides a clear, persistent indication of the active language while preserving the user's selected keyboard theme and avoiding the complexity of maintaining a complete theme for every language.

Changes

• Add a spacebar color setting for each configured language.
• Provide preset colors and custom six- or eight-digit hexadecimal colors.
• Apply the configured color when the active language changes.
• Refresh the spacebar immediately when its color setting changes.
• Select a contrasting foreground color for the language label.
• Keep new labels and color names in translatable string resources.

Screenshots

Per-language setting	Color picker	Active keyboard

Validation

• ./gradlew --no-daemon compileUnstableDebugJavaWithJavac
• git diff --check upstream/master...HEAD

[abb128]

I like the idea, but these colors seem a bit too bright, I think it should act as a more subtle tint instead of a fully bright color and look good across light and dark themes. I'm not sure if we can make it work well with custom themes, for example the Christmas theme. I also think the hex color input should be removed, as well as the color names, for simplicity (the names will likely be difficult to translate to some languages)

[drdanielbender]

Update after review

Implemented the requested UI and color adjustments in b1e92a16d:

• Added a per-language transparency slider so the selected color can act as a subtle tint across light and dark themes. The default transparency is 50%.
• Removed the custom hex-color input.
• Removed the visible color names from the picker while retaining accessible descriptions.
• Replaced the pink preset with white.

As expected by @abb128 in his comment, the Christmas theme is not supported by this change and does not work with it. From my point of view, this is acceptable because not everyone will use the optional feature that changes the spacebar color based on the selected language. If someone likes to use this feature, they will likely be fine with using another theme.

Per-language setting	Color picker	Active keyboard