Allow user-configurable invoice numbering start

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>

app/src/static/style.css

@@ -595,6 +595,12 @@ body {
     outline-offset: 1px;
 }
 
+.field-hint {
+    margin: 0.35rem 0 0;
+    color: var(--muted-ink);
+    font-size: 0.8rem;
+}
+
 .form-group output {
     min-height: 1.5rem;
     padding: 0.25rem 0;