feat: password change + split account/household settings UI

- New /change-password route: validates current password,
  enforces min 4 chars, requires confirmation match
- update_password() helper in models.py
- Dashboard UI split into two collapsible sections:
  👤 Account Settings (password, ntfy, delete account)
  🏠 Household Settings (admin only: public link, timezone,
     delete household)
- 14 new i18n keys with fr/de translations
- Restored accidentally dropped 'Delete My Account' key

models.py

@@ -195,6 +195,13 @@ def set_public_enabled(household_id: int, enabled: bool) -> None:
 
 # ── Users ─────────────────────────────────────────────────────────────────────
 
+def update_password(user_id: int, new_password_hash: str) -> None:
+    """Update a user's password hash."""
+    with get_db() as db:
+        db.execute("UPDATE users SET password_hash = ? WHERE id = ?",
+                   (new_password_hash, user_id))
+
+
 def get_user_by_id(user_id: int) -> dict | None:
     """Fetch a single user by ID."""
     with get_db() as db: