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>

src/fabledassistant/routes/utils.py

@@ -0,0 +1,17 @@
+from datetime import date
+
+from quart import jsonify
+
+
+def not_found(resource: str = "Item"):
+    return jsonify({"error": f"{resource} not found"}), 404
+
+
+def parse_iso_date(value: str | None, field: str = "date"):
+    """Parse an ISO date string. Returns a date, None, or a (response, 400) tuple."""
+    if not value:
+        return None
+    try:
+        return date.fromisoformat(value)
+    except ValueError:
+        return jsonify({"error": f"Invalid {field} format. Use YYYY-MM-DD."}), 400