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/app/globals.css

@@ -114,6 +114,22 @@
   color-scheme: dark;
 }
 
+/* ============================================================
+   Fix native select dropdowns in dark mode
+   ============================================================ */
+select {
+  color-scheme: light;
+}
+
+.dark select {
+  color-scheme: dark;
+}
+
+.dark select option {
+  background-color: var(--color-surface);
+  color: var(--color-text-main);
+}
+
 @theme inline {
   /* Brand scale */
   --color-brand-50: var(--color-brand-50);