style: global readability/theme pass
- Sharpened font stack in index.css, removed softer Georgia digit font for UI text/money
- Tuned dark-mode tokens: clearer foreground, brighter muted text, stronger borders, defined cards
- Updated UI primitives: cards, buttons, inputs, selects, tables, badges
- Cleaned up bills rows, mobile bill rows, tracker dismiss, snowball icons, summary/category/health/analytics money values, import/export status icons
- Reduced fuzzy uppercase label spacing globally
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