feat: replace raw error messages with structured error codes

Backend now returns { code: ErrorCode, message: string } for all errors,
making it language-agnostic. Frontend maps codes to localised strings via
Paraglide (en/de/es), so translations live in messages/*.json.

- Add ErrorCode enum and DomainException with static factory methods
- Update GlobalExceptionHandler to return ErrorResponse(code, message)
- Replace ResponseStatusException throughout controllers/services/aspects
- Add frontend errors.ts with parseBackendError() and getErrorMessage()
- getErrorMessage() delegates to Paraglide m.error_*() functions
- Add error_* keys to messages/en.json, de.json, es.json
- Update all page.server.ts files to use the new error utilities
- Fix hardcoded localhost URLs in admin and login pages
- Fix missing baseUrl in deleteTag action

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

frontend/src/routes/documents/[id]/+page.server.ts

@@ -1,37 +1,24 @@
 import { error, redirect } from '@sveltejs/kit';
 import { env } from '$env/dynamic/private';
+import { parseBackendError, getErrorMessage } from '$lib/errors';
 
 export async function load({ params, fetch }) {
     const { id } = params;
-
-
     const baseUrl = env.API_INTERNAL_URL || 'http://localhost:8080';
 
     try {
         const res = await fetch(`${baseUrl}/api/documents/${id}`);
 
-        if (res.status === 404) {
-            throw error(404, 'Dokument nicht gefunden');
-        }
-
-        if (res.status === 401) {
-            throw redirect(302, '/login');
-        }
+        if (res.status === 401) throw redirect(302, '/login');
 
         if (!res.ok) {
-            console.error(`Backend Fehler (${res.status}):`, res.statusText);
-            throw error(500, 'Fehler beim Laden des Dokuments');
+            const backendError = await parseBackendError(res);
+            throw error(res.status, getErrorMessage(backendError?.code));
         }
 
-        const document = await res.json();
-
-        return {
-            document
-        };
+        return { document: await res.json() };
     } catch (e) {
-        // Fehlerbehandlung
-        if (e.status) throw e; // Redirects und HttpErrors durchlassen
-        console.error("Ladefehler:", e);
-        throw error(500, 'Verbindung zum Server fehlgeschlagen');
+        if (e.status) throw e;
+        throw error(500, getErrorMessage('INTERNAL_ERROR'));
     }
 }

frontend/src/routes/documents/[id]/edit/+page.server.ts

@@ -1,47 +1,48 @@
-import { error, redirect } from '@sveltejs/kit';
+import { error, fail, redirect } from '@sveltejs/kit';
 import { env } from '$env/dynamic/private';
+import { parseBackendError, getErrorMessage } from '$lib/errors';
 
 export async function load({ params, fetch }) {
     const { id } = params;
-    
-
-    const baseUrl = 'http://localhost:8080';
+    const baseUrl = env.API_INTERNAL_URL || 'http://localhost:8080';
 
     try {
-        // Parallel Dokument und Personen laden
         const [docRes, personsRes] = await Promise.all([
             fetch(`${baseUrl}/api/documents/${id}`),
             fetch(`${baseUrl}/api/persons`)
         ]);
 
-        if (!docRes.ok) throw error(docRes.status, 'Dokument nicht gefunden');
-        if (!personsRes.ok) throw error(personsRes.status, 'Personen konnten nicht geladen werden');
+        if (!docRes.ok) {
+            const backendError = await parseBackendError(docRes);
+            throw error(docRes.status, getErrorMessage(backendError?.code));
+        }
+        if (!personsRes.ok) {
+            throw error(personsRes.status, getErrorMessage('INTERNAL_ERROR'));
+        }
 
         return {
             document: await docRes.json(),
             persons: await personsRes.json()
         };
     } catch (e) {
-        console.error(e);
-        throw error(500, 'Ladefehler');
+        if (e.status) throw e;
+        throw error(500, getErrorMessage('INTERNAL_ERROR'));
     }
 }
 
 export const actions = {
     default: async ({ request, params, fetch }) => {
         const baseUrl = env.API_INTERNAL_URL || 'http://localhost:8080';
-
         const formData = await request.formData();
 
-        // Sende den FormData Request direkt an das Spring Backend weiter
-        // (Spring kann Multipart verarbeiten)
         const res = await fetch(`${baseUrl}/api/documents/${params.id}`, {
-            method: "PUT",
+            method: 'PUT',
             body: formData
         });
 
         if (!res.ok) {
-            return { success: false, message: 'Speichern fehlgeschlagen' };
+            const backendError = await parseBackendError(res);
+            return fail(res.status, { error: getErrorMessage(backendError?.code) });
         }
 
         throw redirect(303, `/documents/${params.id}`);