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/composables/useListKeyboardNavigation.ts

@@ -0,0 +1,39 @@
+import { ref, onMounted, onUnmounted } from "vue";
+import type { Ref } from "vue";
+
+export function useListKeyboardNavigation<T extends { id: number }>(
+  items: Ref<T[]>,
+  navigateTo: (item: T) => void,
+  rowSelector = ".kb-active-item",
+  enabled?: Ref<boolean>,
+) {
+  const activeIndex = ref(-1);
+
+  function onKeydown(e: KeyboardEvent) {
+    if (enabled && !enabled.value) 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 = items.value.length;
+    if (e.key === "j") {
+      e.preventDefault();
+      activeIndex.value = Math.min(activeIndex.value + 1, count - 1);
+      document.querySelector(rowSelector)?.scrollIntoView({ block: "nearest" });
+    } else if (e.key === "k") {
+      e.preventDefault();
+      activeIndex.value = Math.max(activeIndex.value - 1, 0);
+      document.querySelector(rowSelector)?.scrollIntoView({ block: "nearest" });
+    } else if (e.key === "Enter" && activeIndex.value >= 0) {
+      const item = items.value[activeIndex.value];
+      if (item) navigateTo(item);
+    }
+  }
+
+  onMounted(() => document.addEventListener("keydown", onKeydown));
+  onUnmounted(() => document.removeEventListener("keydown", onKeydown));
+
+  return { activeIndex };
+}