fix: resolve modal header scroll-behind issue on iOS PWA (closes #50)

position: sticky on .modal-panel__header failed on iOS WebKit when the
scroll container had padding-top applied (drag-handle spacing). Restructured
modal layout: .modal-panel is now a flex-column with overflow:hidden and
.modal-panel__body handles scrolling (overflow-y:auto, flex:1). The header
is a non-scrolled flex sibling, so it stays visible without sticky. Updated
swipe-to-close to read .modal-panel__body scroll position.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/styles/layout.css

@@ -648,7 +648,9 @@
   background-color: var(--color-surface);
   width: 100%;
   max-height: 90dvh;
-  overflow-y: auto;
+  overflow: hidden;
+  display: flex;
+  flex-direction: column;
   border-radius: var(--radius-lg) var(--radius-lg) 0 0;
   border: 1px solid var(--color-border-subtle);
   box-shadow: var(--shadow-lg);
@@ -673,10 +675,8 @@
   justify-content: space-between;
   padding: var(--space-4);
   border-bottom: 1px solid var(--color-border-subtle);
-  position: sticky;
-  top: 0;
   background-color: var(--color-surface);
-  z-index: 1;
+  flex-shrink: 0;
 }
 
 .modal-panel__title {
@@ -706,6 +706,10 @@
   display: flex;
   flex-direction: column;
   gap: var(--space-4);
+  flex: 1;
+  min-height: 0;
+  overflow-y: auto;
+  -webkit-overflow-scrolling: touch;
 }
 
 /* Modal-Content-Grids (z. B. 2 Spalten in Formularen) */