i18n: migrate from German-first to English-first default locale

Switch defaultLocale from "de" to "en" and make English the code-level
language throughout. All ~145 hardcoded German strings translated:

- API error messages (52 routes) now return English
- Zod validation messages in English
- Classification/analytics functions return English categories
- Notification type labels in English
- Component strings use i18n t() calls (heatmap, mood chart,
  medication form, intake history, password strength, insights card)
- ~50 new i18n keys in both de.json and en.json

Route migration:
- /stimmung → /mood, /zielwerte → /targets (new pages)
- 301 redirects in proxy.ts for legacy URLs
- Sidebar + bottom nav updated

Bump version to 1.0.1.

src/app/api/admin/data/route.ts

@@ -30,11 +30,11 @@ export const DELETE = apiHandler(async (request: NextRequest) => {
     const body = await request.json();
     confirm = typeof body?.confirm === "string" ? body.confirm : "";
   } catch {
-    return apiError("Ungültige Anfrage", 422);
+    return apiError("Invalid request", 422);
   }
 
   if (confirm !== "DELETE ALL") {
-    return apiError("Bestätigung fehlt", 422);
+    return apiError("Confirmation missing", 422);
   }
 
   const result = await prisma.$transaction(async (tx) => {