fix(modal): replace native prompt() with custom modal dialogs

Native browser prompt() is unreliable on mobile browsers and PWAs,
often requiring multiple clicks to close. Replace all prompt() calls
with custom promptModal() and selectModal() functions that use the
existing modal system with proper focus management and animations.

Affected pages: shopping (create/rename list), tasks (add subtask),
meals (choose shopping list).

Fixes #12

package.json

@@ -1,6 +1,6 @@
 {
   "name": "oikos",
-  "version": "0.8.0",
+  "version": "0.8.1",
   "description": "Self-hosted family planner - calendar, tasks, shopping, meal planning, budget and more. Private, open-source, no subscription.",
   "main": "server/index.js",
   "type": "module",