fix: keep layout always in DOM via v-show to avoid Firefox click dispatch bug

Replace v-if/v-else-if/v-else auth guard with v-if on LoginPage and
v-show on the layout div. Firefox fails to dispatch clicks to children
after a DOM node is inserted into a display:flex container; using v-show
means the layout is always present (just display:none), so no node
replacement ever occurs in .app-root. Move the forced AccountModal
(mustChangePassword) inside the layout as a first child with v-if.

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

frontend/src/App.vue

@@ -2,13 +2,12 @@
   <div class="app-root">
   <LoginPage v-if="!isAuthenticated" @login="onLogin" />
 
+  <div v-show="isAuthenticated" class="layout">
     <AccountModal
-    v-else-if="mustChangePassword"
+      v-if="mustChangePassword"
       :forced="true"
       @updated="onAccountUpdated"
     />
-
-  <div v-else class="layout">
     <aside class="sidebar">
       <div class="logo">
         <svg width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2">