fix(planner): align protein gradient CSS vars with actual seed tag names

- Rename --gradient-protein-ei → --gradient-protein-eier (tag is 'Eier')
- Add --gradient-protein-kaese for tag 'Käse' (was missing entirely)

The only protein tags in seed data are Käse, Hülsenfrüchte, Eier.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/design-system/tokens.test.ts

@@ -60,7 +60,8 @@ const requiredTokens = [
 	'--gradient-protein-vegetarisch',
 	'--gradient-protein-schwein',
 	'--gradient-protein-lamm',
-	'--gradient-protein-ei',
+	'--gradient-protein-eier',
+	'--gradient-protein-kaese',
 	'--gradient-protein-huelsenfruechte',
 	// Cuisine gradient tokens
 	'--gradient-cuisine-italienisch',