Registration: replace date-of-birth field with a simple 18+ confirmation

The register form collected and validated an actual date of birth just to
throw it away (backend never stored it beyond a nullable column). Per the
roadmap note: don't collect/store DOB right now, just get a client-side
"I am 18 or older" confirmation. RegisterRequest.date_of_birth (str) is now
over_18 (bool); the endpoint rejects over_18=false with 400 same as before.
calculate_age() is kept (unused by /register now) since real DOB collection
is coming back in a future pass and deleting it broke test_auth_age_gate.py's
import for no benefit.

Frontend: DOBEdit LineEdit replaced with an Over18Check CheckBox; api_client.gd's
register() now takes a bool instead of a date string. All Godot callers
(smoke_test_auth/e2e/world_list/main_menu_ui/main_menu_full_flow) and backend
test files updated to match. Full suite verified against a live backend:
61/61 pytest, 6/6 Godot smoke tests (33 individual assertions), all passing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01M6rUEynmv2QmweocAQYARN

scripts/api_client.gd

@@ -178,18 +178,18 @@ func login(username: String, password: String) -> Dictionary:
 
 
 ## Register a new user. On success stores the returned access token.
-## @param date_of_birth ISO date string "YYYY-MM-DD".
+## @param over_18 Must be true; the backend rejects registration otherwise.
 func register(
 	username: String,
 	email: String,
 	password: String,
-	date_of_birth: String
+	over_18: bool
 ) -> Dictionary:
 	var payload := {
 		"username": username,
 		"email": email,
 		"password": password,
-		"date_of_birth": date_of_birth,
+		"over_18": over_18,
 	}
 	var result := await request(
 		HTTPClient.METHOD_POST,