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/useAutoSave.ts

@@ -0,0 +1,22 @@
+import { onMounted, onUnmounted } from "vue";
+import type { Ref } from "vue";
+
+export function useAutoSave(
+  dirty: Ref<boolean>,
+  saving: Ref<boolean>,
+  saveFn: () => Promise<void>,
+  intervalMs = 5 * 60 * 1000,
+): void {
+  let timer: ReturnType<typeof setInterval> | null = null;
+
+  onMounted(() => {
+    timer = setInterval(async () => {
+      if (!dirty.value || saving.value) return;
+      await saveFn();
+    }, intervalMs);
+  });
+
+  onUnmounted(() => {
+    if (timer !== null) clearInterval(timer);
+  });
+}