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/smoke_test_e2e.gd

@@ -23,7 +23,6 @@ extends SceneTree
 const TEST_USERNAME_BASE := "smoke_test_user"
 const TEST_EMAIL := "smoke@test.com"
 const TEST_PASSWORD := "TestPass123"
-const TEST_DOB := "2000-01-01"
 
 
 func _initialize() -> void:
@@ -70,7 +69,7 @@ func _run() -> void:
 
 	# --- Step 2: Register -----------------------------------------------------
 	print("\n=== Step 2: Register (POST /api/auth/register) ===")
-	var reg: Dictionary = await api.register(username, email, TEST_PASSWORD, TEST_DOB)
+	var reg: Dictionary = await api.register(username, email, TEST_PASSWORD, true)
 	var reg_token := _extract_token(reg)
 	var reg_ok: bool = (
 		reg.ok