Move history to sidebar, simplify task assist, add content deduplication

- VersionHistorySection.vue: new sidebar component — collapsible timestamp list,
  click any snapshot to see inline diff + Restore/Back; replaces the modal
- NoteEditorView: remove History toolbar button + HistoryPanel modal, add
  VersionHistorySection at bottom of sidebar
- TaskEditorView: remove floating overlay assist panel + toggle button; add
  Writing Assistant section in sidebar matching note editor (scope selector,
  instruction textarea, generate/proofread/accept/reject); main area now shows
  streaming preview and DiffView like note editor; add VersionHistorySection
- note_versions.py: add content deduplication before time-gap check — identical
  body + title + tags skips snapshot regardless of interval (git semantics)

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

frontend/src/views/NoteEditorView.vue

@@ -18,7 +18,7 @@ import TagInput from "@/components/TagInput.vue";
 import ProjectSelector from "@/components/ProjectSelector.vue";
 import MilestoneSelector from "@/components/MilestoneSelector.vue";
 import DiffView from "@/components/DiffView.vue";
-import HistoryPanel from "@/components/HistoryPanel.vue";
+import VersionHistorySection from "@/components/VersionHistorySection.vue";
 import ConfirmDialog from "@/components/ConfirmDialog.vue";
 
 const route = useRoute();
@@ -35,7 +35,6 @@ const dirty = ref(false);
 const saving = ref(false);
 const showPreview = ref(false);
 const sidebarOpen = ref(true);
-const showHistory = ref(false);
 const editorRef = ref<InstanceType<typeof TiptapEditor> | null>(null);
 const titleRef = ref<HTMLInputElement | null>(null);
 const tiptapEditor = computed<Editor | null>(() => {
@@ -326,7 +325,6 @@ onUnmounted(() => assist.clearSelection());
           {{ saving ? "Saving..." : "Save" }}
         </button>
         <button v-if="isEditing" class="btn-delete" @click="remove">Delete</button>
-        <button v-if="isEditing" class="btn-history" @click="showHistory = true">History</button>
         <WordCount :body="body" />
       </div>
       <input
@@ -513,6 +511,13 @@ onUnmounted(() => assist.clearSelection());
             </template>
           </div>
 
+          <VersionHistorySection
+            v-if="noteId"
+            :note-id="noteId"
+            :current-body="body"
+            @restore="(b, t) => { body = b; tags = t; markDirty(); }"
+          />
+
         </div><!-- /sidebar-content -->
       </aside>
     </div><!-- /note-body -->
@@ -527,15 +532,6 @@ onUnmounted(() => assist.clearSelection());
       >✨ Assist</button>
     </teleport>
 
-    <!-- History panel -->
-    <HistoryPanel
-      v-if="showHistory && noteId"
-      :note-id="noteId"
-      :current-body="body"
-      @restore="(b: string, t: string[]) => { body = b; tags = t; markDirty(); }"
-      @close="showHistory = false"
-    />
-
     <!-- Delete confirmation -->
     <ConfirmDialog
       v-if="showDeleteConfirm"
@@ -738,22 +734,6 @@ onUnmounted(() => assist.clearSelection());
   letter-spacing: 0.05em;
 }
 
-/* History button */
-.btn-history {
-  padding: 0.45rem 1rem;
-  background: none;
-  border: 1px solid var(--color-border);
-  border-radius: var(--radius-sm);
-  cursor: pointer;
-  font-size: 0.875rem;
-  color: var(--color-text-secondary);
-  font-family: inherit;
-}
-.btn-history:hover {
-  border-color: var(--color-primary);
-  color: var(--color-primary);
-}
-
 /* Narrow screen: sidebar collapses */
 @media (max-width: 720px) {
   .note-body { flex-direction: column; overflow-y: auto; overflow-x: hidden; }