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'));
     }
 }