Support alpha in chrome background hex

[lawrencecchen]

Summary

• accept #RRGGBBAA in BonsplitConfiguration.Appearance.ChromeColors.backgroundHex
• keep existing #RRGGBB behavior unchanged
• add a regression test that verifies parsed RGBA components for pane background

Context

• needed by ghostty transparency not supported cmux#263 so cmux can pass Ghostty background opacity through pane/split chrome layers

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Blend alpha colors before deriving text contrast

Allowing 8-digit hex values here introduces translucent chrome colors, but text/icon contrast is still computed from raw RGB in effectiveTextColor/isBonsplitLightColor without considering alpha compositing against the fallback background. With inputs like #00000000 (or any low-alpha dark color) on a light window, the code picks white foreground text even though the rendered background is effectively light, making tab labels hard to read.

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