Revise dark mode color scheme with lighter dark tones

[MessiasLima]

Description

The current dark-mode color palette is too dark, with background and surface colors near black (#111413, #141313), making the UI feel heavy and reducing readability. Create a new set of dark color tokens that use a softer, less aggressive dark palette while maintaining proper contrast ratios for accessibility.

• Scope and constraints
  • Only dark-mode color values in Color.kt should change; light scheme stays untouched.
  • All dark-color variable names (*Dark) must remain the same — only the hex values change.
  • Must maintain WCAG AA contrast against text/icon colors.
  • The darkScheme in Theme.kt references the same *Dark variables, so no changes needed there.
  • Components use MaterialTheme.colorScheme.* tokens and will pick up the new colors automatically.

Scope

In scope: Updating dark color hex values in Color.kt (primaryDark through surfaceContainerHighestDark).

Out of scope: Light scheme, typography, shapes, component-level changes, or adding new color tokens.

Acceptance criteria

• Dark background/surface colors are updated to a softer dark gray (not near-black) in Color.kt.
• All on*Dark text/icon colors are adjusted if needed to maintain WCAG AA contrast.
• App renders correctly in dark mode on all platforms (Android, iOS, JVM).
• No visual regressions in components using MaterialTheme.colorScheme tokens in dark mode.

Additional information

The current dark colors are defined in foliary/src/commonMain/kotlin/dev/appoutlet/foliary/core/ui/theme/Color.kt:40-74. The assignee should create a cohesive dark palette from scratch with appropriate contrast.