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/useFloatingAssist.ts

@@ -0,0 +1,34 @@
+import { ref } from "vue";
+
+type Selection = { text: string; start: number; end: number };
+
+export function useFloatingAssist(onRequest: (sel: Selection) => void) {
+  const floatingAssist = ref({ show: false, top: 0, left: 0 });
+  const pendingSelection = ref<Selection | null>(null);
+
+  function onSelectionChange(payload: Selection) {
+    if (payload.text.trim().length > 0) {
+      const sel = window.getSelection();
+      if (sel && sel.rangeCount > 0) {
+        const rect = sel.getRangeAt(0).getBoundingClientRect();
+        floatingAssist.value = {
+          show: true,
+          top: rect.top - 44,
+          left: rect.left + rect.width / 2,
+        };
+        pendingSelection.value = payload;
+      }
+    } else {
+      floatingAssist.value.show = false;
+      pendingSelection.value = null;
+    }
+  }
+
+  function handleInlineAssist() {
+    if (!pendingSelection.value) return;
+    floatingAssist.value.show = false;
+    onRequest(pendingSelection.value);
+  }
+
+  return { floatingAssist, onSelectionChange, handleInlineAssist };
+}