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>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "oikos",
-  "version": "0.20.10",
+  "version": "0.20.11",
   "description": "Self-hosted family planner - calendar, tasks, shopping, meal planning, budget and more. Private, open-source, no subscription.",
   "main": "server/index.js",
   "type": "module",