Add version footer, task history UI, and note version retention policy

- App.vue: quiet version footer (fetches /api/version, 0.45 opacity)
- TaskEditorView: History button + HistoryPanel integration (mirrors NoteEditorView)
- note_versions.py: raise MAX_VERSIONS 20→50; add 5-min minimum interval gate
  to prevent autosave exhausting history slots

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

frontend/src/App.vue

@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { onMounted, onUnmounted, watch } from "vue";
+import { onMounted, onUnmounted, ref, watch } from "vue";
 import { useRouter } from "vue-router";
 import AppHeader from "@/components/AppHeader.vue";
 import ToastNotification from "@/components/ToastNotification.vue";
@@ -8,10 +8,12 @@ import { useShortcuts } from "@/composables/useShortcuts";
 import { useAuthStore } from "@/stores/auth";
 import { useChatStore } from "@/stores/chat";
 import { useSettingsStore } from "@/stores/settings";
+import { apiGet } from "@/api/client";
 
 useTheme();
 
 const router = useRouter();
+const appVersion = ref("dev");
 const authStore = useAuthStore();
 const chatStore = useChatStore();
 const settingsStore = useSettingsStore();
@@ -123,6 +125,12 @@ onMounted(async () => {
   if (authStore.isAuthenticated) {
     startAppServices();
   }
+  try {
+    const data = await apiGet<{ version: string }>("/api/version");
+    appVersion.value = data.version;
+  } catch {
+    // silent — version display is non-critical
+  }
 });
 
 watch(
@@ -150,6 +158,7 @@ onUnmounted(() => {
       <div id="main-content" class="app-content">
         <router-view />
       </div>
+      <footer class="app-footer">v{{ appVersion }}</footer>
     </div>
 
     <!-- Keyboard shortcuts overlay -->
@@ -299,6 +308,18 @@ onUnmounted(() => {
   overflow: hidden;
 }
 
+/* Version footer */
+.app-footer {
+  flex-shrink: 0;
+  text-align: center;
+  padding: 0.2rem 0;
+  font-size: 0.68rem;
+  color: var(--color-text-muted);
+  opacity: 0.45;
+  user-select: none;
+  letter-spacing: 0.03em;
+}
+
 /* Shortcuts overlay */
 .shortcuts-overlay {
   position: fixed;