Fix automatic readable text contrast for public and admin themes.

Derive admin/public text colors from WCAG contrast, unify ThemeVars CSS emission, and keep navbar overrides in sync with readable vars.

Co-authored-by: Cursor <[email protected]>

src/app/admin/theme/color-field.tsx

@@ -73,7 +73,17 @@ export function ColorField({
 				type="color"
 				name={name}
 				value={val}
-				onChange={(e) => setVal(e.target.value)}
+				onChange={(e) => {
+					const next = e.target.value;
+					if (bgValue) {
+						const nextRatio = contrastRatio(next, bgValue);
+						if (nextRatio != null && nextRatio < 4.5) {
+							setVal(readableOn(bgValue));
+							return;
+						}
+					}
+					setVal(next);
+				}}
 				style={{
 					width: 42,
 					height: 36,