DRY refactoring pass: shared mixins, route helpers, composables, CSS

Backend:
- models/base.py: TimestampMixin + CreatedAtMixin; applied to all 10+ models
- routes/utils.py: not_found() + parse_iso_date() helpers; used across all route files
- routes/milestones.py: _milestone_dict() helper replaces 5 repeated to_dict + progress blocks

Frontend:
- 5 new composables: useAutoSave, useEditorGuards, useTagSuggestions,
  useFloatingAssist, useListKeyboardNavigation
- ConfirmDialog.vue: reusable confirm modal replacing inline <teleport> blocks
- editor-shared.css: sidebar CSS consolidated from both editor views
- viewer-shared.css: context-bar/breadcrumb CSS consolidated from both viewer views
- NoteEditorView + TaskEditorView: ~120 lines each replaced with composable calls;
  duplicate scoped sidebar CSS removed
- NotesListView + TasksListView: inline keyboard-nav replaced with composable
- NoteViewerView + TaskViewerView: duplicate context-bar CSS removed

No behaviour changes. Net: -634 lines, +237 lines across 31 files.

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

frontend/src/views/TasksListView.vue

@@ -4,6 +4,7 @@ import { useRoute, useRouter } from "vue-router";
 import { useTasksStore } from "@/stores/tasks";
 import type { Task, TaskStatus } from "@/types/task";
 import { apiGet } from "@/api/client";
+import { useListKeyboardNavigation } from "@/composables/useListKeyboardNavigation";
 import SearchBar from "@/components/SearchBar.vue";
 import TaskCard from "@/components/TaskCard.vue";
 import TagPill from "@/components/TagPill.vue";
@@ -16,33 +17,11 @@ const router = useRouter();
 const store = useTasksStore();
 
 const searchBarRef = ref<{ focus: () => void } | null>(null);
-const activeIndex = ref(-1);
 
 function onFocusSearch() {
   searchBarRef.value?.focus();
 }
 
-function onKeydown(e: KeyboardEvent) {
-  if (viewMode.value !== "flat") return;
-  const el = document.activeElement;
-  const tag = el ? (el as HTMLElement).tagName : "";
-  const isInput = tag === "INPUT" || tag === "TEXTAREA" || (el as HTMLElement)?.isContentEditable;
-  if (isInput || e.ctrlKey || e.metaKey || e.altKey) return;
-  const count = store.tasks.length;
-  if (e.key === "j") {
-    e.preventDefault();
-    activeIndex.value = Math.min(activeIndex.value + 1, count - 1);
-    document.querySelector(".kb-active-item")?.scrollIntoView({ block: "nearest" });
-  } else if (e.key === "k") {
-    e.preventDefault();
-    activeIndex.value = Math.max(activeIndex.value - 1, 0);
-    document.querySelector(".kb-active-item")?.scrollIntoView({ block: "nearest" });
-  } else if (e.key === "Enter" && activeIndex.value >= 0) {
-    const task = store.tasks[activeIndex.value];
-    if (task) router.push(`/tasks/${task.id}`);
-  }
-}
-
 const viewMode = ref<ViewMode>(
   (localStorage.getItem("fabled-tasks-view-mode") as ViewMode) ?? "flat"
 );
@@ -60,6 +39,14 @@ function setViewMode(mode: ViewMode) {
   store.refresh();
 }
 
+const isFlat = computed(() => viewMode.value === "flat");
+const { activeIndex } = useListKeyboardNavigation(
+  computed(() => store.tasks),
+  (task) => router.push(`/tasks/${task.id}`),
+  ".kb-active-item",
+  isFlat,
+);
+
 // Project map for group labels and task breadcrumbs
 const projectMap = ref<Map<number, string>>(new Map());
 
@@ -119,12 +106,10 @@ onMounted(async () => {
   }
   await Promise.all([store.refresh(), loadProjects()]);
   document.addEventListener("shortcut:focus-search", onFocusSearch);
-  document.addEventListener("keydown", onKeydown);
 });
 
 onUnmounted(() => {
   document.removeEventListener("shortcut:focus-search", onFocusSearch);
-  document.removeEventListener("keydown", onKeydown);
 });
 
 watch(