fix(colors): stop defaulting to the unknown color name 'gray'

[z1pp090]

Fixes #540.

Problem

gray is not a COLOR_MAP entry (the neutral one is brightBlack, src/utils/colors.ts:29), and getColorAnsiCode returns '' on a lookup miss. Every site that fell back to 'gray' therefore rendered with no color at all:

• src/utils/renderer.ts — let separatorColor = widget.color ?? 'gray', the default path for every separator with no explicit color while inheritSeparatorColors is off (the shipped default line has three of them).
• getDefaultColor() of GitSha, TerminalWidth and Version.

The Ink <Text color='gray'> usages inside the TUI are unrelated: Ink resolves that name itself.

Fix

Swap the four sites to brightBlack. No new color name is introduced, so saved settings and the color menu are unaffected.

Tests

src/utils/__tests__/default-colors.test.ts:

• every widget in WIDGET_MANIFEST returns a default color that exists in COLOR_MAP (fails on main with git-sha -> 'gray');
• a separator with no color is rendered with the bright-black SGR code instead of bare (fails on main: | comes out uncolored).

bun test and bun run lint green.

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