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/login/+page.server.ts

@@ -1,4 +1,6 @@
 import { fail, redirect, type Actions } from '@sveltejs/kit';
+import { env } from '$env/dynamic/private';
+import { getErrorMessage } from '$lib/errors';
 
 export const actions = {
     login: async ({ request, cookies, fetch }) => {
@@ -17,7 +19,8 @@ export const actions = {
         try {
             // Test-Request an das Backend (z.B. an den Upload-Endpunkt oder einen speziellen /me Endpunkt)
             // Wir nutzen hier http://localhost:8080, da beide Container im selben Netz sind (oder localhost im DevContainer)
-            const response = await fetch('http://localhost:8080/api/users/me', {
+            const baseUrl = env.API_INTERNAL_URL || 'http://localhost:8080';
+            const response = await fetch(`${baseUrl}/api/users/me`, {
                 method: 'GET',
                 headers: {
                     'Authorization': authHeader
@@ -25,11 +28,11 @@ export const actions = {
             });
 
             if (response.status === 401 || response.status === 403) {
-                return fail(401, { error: 'Ungültige Zugangsdaten.' });
+                return fail(401, { error: getErrorMessage('UNAUTHORIZED') });
             }
 
             if (!response.ok) {
-                return fail(500, { error: 'Serverfehler beim Login.' });
+                return fail(500, { error: getErrorMessage('INTERNAL_ERROR') });
             }
 
             // Login erfolgreich! Wir speichern den Header in einem Cookie.
@@ -42,9 +45,9 @@ export const actions = {
                 maxAge: 60 * 60 * 24 // 1 Tag
             });
 
-        } catch (error) {
-            console.error(error);
-            return fail(500, { error: 'Verbindung zum Backend fehlgeschlagen.' });
+        } catch (e) {
+            console.error(e);
+            return fail(500, { error: getErrorMessage('INTERNAL_ERROR') });
         }
 
         // Weiterleitung zur Startseite