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/users/[id]/reset-password/route.ts

@@ -32,16 +32,16 @@ export const POST = apiHandler(async (
   const { password } = body as { password?: string };
 
   if (!password || typeof password !== "string") {
-    return apiError("Passwort erforderlich", 422);
+    return apiError("Password required", 422);
   }
 
   const target = await prisma.user.findUnique({ where: { id } });
-  if (!target) return apiError("Benutzer nicht gefunden", 404);
+  if (!target) return apiError("User not found", 404);
 
   const strength = checkPasswordStrength(password, [target.username]);
   if (!strength.isAcceptable) {
     return apiError(
-      strength.feedback[0] || "Passwort zu schwach (Score < 3)",
+      strength.feedback[0] || "Password too weak (score < 3)",
       422,
     );
   }

src/app/api/admin/users/[id]/route.ts

@@ -30,13 +30,13 @@ export const PUT = apiHandler(async (
   }
 
   const target = await prisma.user.findUnique({ where: { id } });
-  if (!target) return apiError("Benutzer nicht gefunden", 404);
+  if (!target) return apiError("User not found", 404);
 
   // Prevent removing the last admin
   if (parsed.data.role === "USER" && target.role === "ADMIN") {
     const adminCount = await prisma.user.count({ where: { role: "ADMIN" } });
     if (adminCount <= 1) {
-      return apiError("Der letzte Admin kann nicht degradiert werden", 400);
+      return apiError("The last admin cannot be demoted", 400);
     }
   }