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/useEditorGuards.ts

@@ -0,0 +1,33 @@
+import { onMounted, onUnmounted } from "vue";
+import { onBeforeRouteLeave } from "vue-router";
+import type { Ref } from "vue";
+
+export function useEditorGuards(
+  dirty: Ref<boolean>,
+  saveFn: () => void | Promise<void>,
+): void {
+  function onKeydown(e: KeyboardEvent) {
+    if ((e.ctrlKey || e.metaKey) && e.key === "s") {
+      e.preventDefault();
+      saveFn();
+    }
+  }
+
+  function onBeforeUnload(e: BeforeUnloadEvent) {
+    if (dirty.value) e.preventDefault();
+  }
+
+  onMounted(() => {
+    document.addEventListener("keydown", onKeydown);
+    window.addEventListener("beforeunload", onBeforeUnload);
+  });
+
+  onUnmounted(() => {
+    document.removeEventListener("keydown", onKeydown);
+    window.removeEventListener("beforeunload", onBeforeUnload);
+  });
+
+  onBeforeRouteLeave(() => {
+    if (dirty.value) return confirm("You have unsaved changes. Leave anyway?");
+  });
+}