Add a per-user invoice_starting_number setting (default 1) instead of always starting invoice numbering at 001. Adds /api/settings endpoints, a user_settings table, and a Číslování faktur panel in the account menu where the user can pick the starting number for a year with no invoices yet. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>