chore(ui): unify heading weight, card cadence, and label-input gap across settings and admin

Sub-headings in the settings + admin section trees were mixing
font-medium and font-semibold; the password-change dialog used a
1.5-unit label-input gap while every other Settings form runs at 2.
Standardise on font-semibold for every h2 and h3, the card-internal
cadence to space-y-4 on the AI provider and thresholds-editor cards,
and lift the password-dialog field gaps to space-y-2.

src/components/admin/user-management-section.tsx

@@ -417,7 +417,7 @@ export function UserManagementSection() {
       {/* Edit Dialog */}
       {editingUser && (
         <div className="bg-muted/80 mt-4 rounded-lg p-4">
-          <h3 className="mb-3 text-sm font-medium">
+          <h3 className="mb-3 text-sm font-semibold">
             {t("admin.editUserTitle", { name: editingUser.username })}
           </h3>
           <div className="space-y-3">
@@ -484,7 +484,7 @@ export function UserManagementSection() {
       {/* Password Reset Dialog */}
       {resetUser && (
         <div className="bg-muted/80 mt-4 rounded-lg p-4">
-          <h3 className="mb-3 text-sm font-medium">
+          <h3 className="mb-3 text-sm font-semibold">
             {t("admin.resetPasswordTitle", { name: resetUser.username })}
           </h3>
           <div className="space-y-3">