fix: improve dropdown text readability in dark theme on usage page (#997)

- Add global CSS rules for select elements in dark mode
- Use color-scheme property to signal dark mode to browser
- Explicitly style option elements with dark theme colors
- Fix UsageStats dropdown to use correct CSS variables (bg-surface, text-text-main)

Fixes dropdown text being unreadable in dark theme on usage page:
- Provider filter dropdown
- Table view selector (Model/Account/API Key/Endpoint)
- Pagination page size selector

Tested in Chrome and Firefox with both light and dark themes.

src/shared/components/Pagination.js

@@ -63,6 +63,7 @@ export default function Pagination({
                 "text-sm text-text-main focus:outline-none focus:ring-2 focus:ring-primary/20",
                 "cursor-pointer"
               )}
+              style={{ colorScheme: 'auto' }}
             >
               {[10, 20, 50].map((size) => (
                 <option key={size} value={size}>