## Issue
Fixes #2962 - Improve the background color at hovering and MOST OF ALL
for selected projects.

## Changes
- Increased dark mode sidebar-accent background color contrast from
15.9% to 35% lightness
- Updated both OKLch and HSL color formats for consistency
- Selected items are now clearly distinguishable from unselected items

### Dark mode improvements:
- OKLch: `oklch(0.35 0 0)` instead of `oklch(0.269 0 0)`
- HSL: `240 5% 35%` instead of `240 3.7% 15.9%`

This dramatically improves visibility for users with less acute vision,
addressing the accessibility concern raised in the issue.

## Testing
Added comprehensive E2E test to verify:
- Selected sidebar items have visually distinct background colors
- Hover states provide proper visual feedback
- Good contrast between selected, unselected, and hover states
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