feat: add create person feature via web interface

- Backend: new POST /api/persons endpoint in PersonController
- Frontend: new /persons/new route with Vorname/Nachname/Alias form,
  redirects to the new person's detail page on success
- Persons list: subtle '+ Neue Person' link below the page title

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

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

@@ -44,6 +44,21 @@ public class PersonController {
         return documentRepository.findBySenderId(id);
     }
 
+    @PostMapping
+    public ResponseEntity<Person> createPerson(@RequestBody Map<String, String> body) {
+        String firstName = body.get("firstName");
+        String lastName = body.get("lastName");
+        if (firstName == null || firstName.isBlank() || lastName == null || lastName.isBlank()) {
+            throw new ResponseStatusException(HttpStatus.BAD_REQUEST, "Vor- und Nachname sind Pflichtfelder");
+        }
+        Person person = Person.builder()
+                .firstName(firstName.trim())
+                .lastName(lastName.trim())
+                .alias(body.get("alias"))
+                .build();
+        return ResponseEntity.ok(personRepository.save(person));
+    }
+
     @PutMapping("/{id}")
     public ResponseEntity<Person> updatePerson(@PathVariable UUID id, @RequestBody Map<String, String> body) {
         String firstName = body.get("firstName");