🎨 Palette: [UX improvement] Fix invisible text on button hover

[0m364]

💡 What: The .button:hover CSS class was updated to explicitly define color: var(--secondary-color); and border-color: var(--secondary-color);, along with a 2px solid transparent border on the base .button class. Transition properties were updated to smoothly animate these color changes.
🎯 Why: Previously, the button background inverted to black #000000 on hover, but the text color (which was also black) did not update. This resulted in invisible text and icons, breaking accessibility and creating a confusing user experience. Adding a transparent border prevents layout shifting on hover while providing a space for a focus-friendly border.
📸 Before/After: No visual regressions. Hover screenshots generated via Playwright.
♿ Accessibility: This directly fixes a WCAG color contrast violation on interactive elements, ensuring text remains visible and legible during hover states. I also added a journal entry to .Jules/palette.md to document this learning.

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PR created automatically by Jules for task 15857262264434400233 started by @0m364

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