feat(fe): add app version display in sidebar footer

- Display app version (v0.1.0 or VITE_APP_VERSION) in sidebar footer
- Hidden when sidebar is collapsed
- Positioned above Help/Settings buttons

This provides system version visibility without requiring footer.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

frontend/src/shared/shell/KsSidebar.vue

@@ -3,6 +3,8 @@ import { ref, computed, onMounted, watch } from 'vue'
 import { useRouter, useRoute } from 'vue-router'
 import { useAnimatedCollapse } from '../composables/useAnimatedCollapse'
 
+const appVersion = import.meta.env.VITE_APP_VERSION ?? '0.1.0'
+
 interface MenuItem {
   id: string
   label: string
@@ -194,6 +196,9 @@ const sidebarClass = computed(() => ({
 
     <!-- Sidebar footer -->
     <div class="ks-sidebar__footer">
+      <div v-if="!collapsed" class="ks-sidebar__version">
+        v{{ appVersion }}
+      </div>
       <button
         type="button"
         class="ks-sidebar__footer-btn"
@@ -370,8 +375,9 @@ const sidebarClass = computed(() => ({
   padding: var(--spacing-3);
   border-top: 1px solid var(--color-border-primary);
   display: flex;
+  flex-direction: column;
   gap: var(--spacing-2);
-  justify-content: center;
+  align-items: center;
 }
 
 .ks-sidebar__footer-btn {