ui: add custom indigo selection highlight color matching dark theme

[Malaybhai11]

Replace the browser's default bright blue text selection color with a custom indigo highlight (rgba(99, 102, 241, 0.4)) that matches the dark premium aesthetic of CommitPulse.

Changes:

• Added ::selection with indigo background matching the existing accent color
• Added ::-moz-selection for Firefox compatibility
• Keeps text white for contrast against dark background

Closes #60

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