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

backend/app/routers/auth.py

@@ -1,7 +1,7 @@
 """Authentication router - login, register, token refresh."""
 
 import uuid
-from datetime import datetime, timedelta, timezone
+from datetime import date, datetime, timedelta, timezone
 
 from fastapi import APIRouter, Depends, HTTPException, status
 from fastapi.security import OAuth2PasswordRequestForm
@@ -105,6 +105,36 @@ def hash_password(password: str) -> str:
     return pwd_context.hash(password)
 
 
+def calculate_age(birth_date: date, today: date | None = None) -> int:
+    """Calculate age in years from a birth date.
+
+    Not called by /register anymore (that endpoint now just trusts the
+    client's over_18 boolean, see RegisterRequest) -- kept as a real,
+    tested utility since date-of-birth storage/verification is coming
+    back in a future pass (real age math, not just a client-side claim,
+    the user explicitly wants to revisit this design). Removing it broke
+    backend/tests/test_auth_age_gate.py's collection entirely (ImportError
+    on `from app.routers.auth import calculate_age`) for zero benefit --
+    restoring it is the lower-risk path over rewriting or deleting 17
+    tests for logic this codebase will need again.
+
+    Args:
+        birth_date: The person's date of birth.
+        today: The reference date. Defaults to today's date (UTC).
+
+    Returns:
+        Age in whole years as of the reference date.
+    """
+    if today is None:
+        today = datetime.now(timezone.utc).date()
+
+    age = today.year - birth_date.year
+    # Adjust if birthday hasn't occurred yet this year
+    if (today.month, today.day) < (birth_date.month, birth_date.day):
+        age -= 1
+    return age
+
+
 def _issue_refresh_token(user_id: int, db: AsyncSession) -> str:
     """Create and persist a refresh token for the given user."""
     refresh_token_value = _create_refresh_token({"sub": str(user_id)})