fix: increase card elevation contrast in dark mode (closes #1067)

[bhavyanjain3004]

Summary

Increases the elevation contrast for dark mode to improve visual hierarchy and readability on non-OLED screens (Closes #1067).

Key Changes:

• Updated src/app/globals.css to increase the contrast of .dark surfaces.
• Adjusted --card to #111111 and --card-muted to #1a1a1a to stand out more clearly against the dark background.
• Increased the visibility of --border to #2a2a2a to provide subtle elevation cues.

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