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/lib/api-handler.ts

@@ -103,7 +103,7 @@ export async function requireAuth(): Promise<{
     : never;
 }> {
   const sessionData = await getSession();
-  if (!sessionData) throw new HttpError(401, "Nicht angemeldet");
+  if (!sessionData) throw new HttpError(401, "Not authenticated");
 
   const evt = getEvent();
   if (evt) {
@@ -131,7 +131,7 @@ export async function requireAdmin(): Promise<{
 }> {
   const sessionData = await requireAuth();
   if (sessionData.user.role !== "ADMIN") {
-    throw new HttpError(403, "Nur Admins erlaubt");
+    throw new HttpError(403, "Admin access required");
   }
   return sessionData;
 }