refactor: migrate all page.server.ts files to typed API client

All server-side fetch calls now go through createApiClient() from
$lib/api.server.ts, which wraps openapi-fetch with the generated OpenAPI
types. This means backend changes are reflected in the frontend after
running npm run generate:api.

- Add stub src/lib/generated/api.ts (replaced by generate:api output)
- Fix GroupController: missing /api prefix and ResponseStatusException
- Root, conversations, persons, documents pages all use typed client
- Error handling uses apiError.code directly (no parseBackendError needed)
- Edit page load uses typed client; PUT action keeps raw fetch (multipart)
- Login keeps raw fetch (explicit Authorization header, not cookie auth)

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

backend/src/main/java/org/raddatz/familienarchiv/controller/GroupController.java

@@ -9,7 +9,8 @@ import org.raddatz.familienarchiv.repository.UserGroupRepository;
 import org.raddatz.familienarchiv.security.Permission;
 import org.raddatz.familienarchiv.security.RequirePermission;
 import org.raddatz.familienarchiv.service.UserService;
-import org.springframework.http.HttpStatus;
+import org.raddatz.familienarchiv.exception.DomainException;
+import org.raddatz.familienarchiv.exception.ErrorCode;
 import org.springframework.http.ResponseEntity;
 import org.springframework.web.bind.annotation.DeleteMapping;
 import org.springframework.web.bind.annotation.GetMapping;
@@ -19,12 +20,11 @@ import org.springframework.web.bind.annotation.PostMapping;
 import org.springframework.web.bind.annotation.RequestBody;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.RestController;
-import org.springframework.web.server.ResponseStatusException;
 
 import lombok.RequiredArgsConstructor;
 
 @RestController
-@RequestMapping("/groups")
+@RequestMapping("/api/groups")
 @RequirePermission(Permission.ADMIN_PERMISSION)
 @RequiredArgsConstructor
 public class GroupController {
@@ -42,7 +42,7 @@ public class GroupController {
     @PatchMapping("/{id}")
     public ResponseEntity<UserGroup> updateGroup(@PathVariable UUID id, @RequestBody GroupDTO dto) {
         UserGroup group = groupRepository.findById(id)
-                .orElseThrow(() -> new ResponseStatusException(HttpStatus.NOT_FOUND));
+                .orElseThrow(() -> DomainException.notFound(ErrorCode.INTERNAL_ERROR, "Group not found: " + id));
 
         if (dto.getName() != null)
             group.setName(dto.getName());
@@ -53,14 +53,9 @@ public class GroupController {
     }
 
     @DeleteMapping("/{id}")
-    public ResponseEntity<?> deleteGroup(@PathVariable UUID id) {
-        try {
-            // Optional: Check if users are assigned before deleting
-            groupRepository.deleteById(id);
-            return ResponseEntity.ok().build();
-        } catch (Exception e) {
-            return ResponseEntity.badRequest().body("Gruppe konnte nicht gelöscht werden.");
-        }
+    public ResponseEntity<Void> deleteGroup(@PathVariable UUID id) {
+        groupRepository.deleteById(id);
+        return ResponseEntity.ok().build();
     }
 
     @GetMapping("")