feat(modals): FormModal gains submitDisabled + subtitle props

submitDisabled greys out the primary (so callers can gate it on validity
instead of a clickable-but-inert button); subtitle renders a muted line under
the title in the header.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/components.css

@@ -255,12 +255,23 @@
   gap: 12px;
 }
 
+.admin-modal-heading {
+  min-width: 0;
+}
+
 .admin-modal-title {
   font-size: 16px;
   font-weight: 700;
   color: var(--text-primary);
 }
 
+.admin-modal-subtitle {
+  margin: 2px 0 0;
+  font-size: 12.5px;
+  font-weight: 500;
+  color: var(--text-secondary);
+}
+
 .admin-modal-close {
   flex-shrink: 0;
   display: inline-flex;

src/admin/components/FormModal.tsx

@@ -15,10 +15,15 @@ export interface FormModalProps {
   onClose: () => void;
   onSubmit?: () => void; // called when user clicks primary button (only if provided)
   title: string;
+  /** Optional muted line shown under the title in the header. */
+  subtitle?: ReactNode;
   children: ReactNode; // modal body (form fields)
   submitLabel?: string; // primary button text
   cancelLabel?: string; // cancel button text
   loading?: boolean;
+  /** Disable the primary (submit) button — e.g. until the form is valid.
+   *  Keeps the button visibly greyed out instead of clickable-but-inert. */
+  submitDisabled?: boolean;
   hideFooter?: boolean; // for "view-only" modals
   size?: "sm" | "md" | "lg"; // optional
   closeOnBackdrop?: boolean; // default true
@@ -35,10 +40,12 @@ export default function FormModal({
   onClose,
   onSubmit,
   title,
+  subtitle,
   children,
   submitLabel = "Uložit",
   cancelLabel = "Zrušit",
   loading = false,
+  submitDisabled = false,
   hideFooter = false,
   size = "md",
   closeOnBackdrop = true,
@@ -107,9 +114,12 @@ export default function FormModal({
             transition={{ duration, ease }}
           >
             <div className="admin-modal-header">
-              <h2 id={titleId} className="admin-modal-title">
-                {title}
-              </h2>
+              <div className="admin-modal-heading">
+                <h2 id={titleId} className="admin-modal-title">
+                  {title}
+                </h2>
+                {subtitle && <p className="admin-modal-subtitle">{subtitle}</p>}
+              </div>
               {!hideCloseButton && (
                 <button
                   type="button"
@@ -175,7 +185,7 @@ export default function FormModal({
                       type="button"
                       onClick={handlePrimaryClick}
                       className="admin-btn admin-btn-primary"
-                      disabled={loading}
+                      disabled={loading || submitDisabled}
                     >
                       {loading ? "Zpracování..." : submitLabel}
                     </button>