fix(ui): add custom text selection color to match site theme

[likitha-codes]

Description

The default browser text selection (that bright blue highlight)
looks out of place on CommitPulse's dark premium design. This PR
adds a simple ::selection rule to globals.css so highlighted text
uses the site's green accent color instead of browser blue.

Closes #60

Pillar

Frontend / UI Enhancement

Checklist

• I have read CONTRIBUTING.md
• My change is minimal and focused on the issue
• No new dependencies added
• Change applies globally with no component modifications

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