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/projects.py

@@ -4,6 +4,7 @@ import logging
 from quart import Blueprint, jsonify, request
 
 from fabledassistant.auth import login_required, get_current_user_id
+from fabledassistant.routes.utils import not_found
 from fabledassistant.services.milestones import list_milestones
 from fabledassistant.services.notes import list_notes
 from fabledassistant.services.projects import (
@@ -52,7 +53,7 @@ async def get_project_route(project_id: int):
     uid = get_current_user_id()
     project = await get_project(uid, project_id)
     if project is None:
-        return jsonify({"error": "Project not found"}), 404
+        return not_found("Project")
     summary = await get_project_summary(uid, project_id)
     data = project.to_dict()
     data["summary"] = summary
@@ -68,7 +69,7 @@ async def update_project_route(project_id: int):
     fields = {k: v for k, v in data.items() if k in allowed}
     project = await update_project(uid, project_id, **fields)
     if project is None:
-        return jsonify({"error": "Project not found"}), 404
+        return not_found("Project")
     return jsonify(project.to_dict())
 
 
@@ -78,7 +79,7 @@ async def delete_project_route(project_id: int):
     uid = get_current_user_id()
     deleted = await delete_project(uid, project_id)
     if not deleted:
-        return jsonify({"error": "Project not found"}), 404
+        return not_found("Project")
     return "", 204
 
 
@@ -88,7 +89,7 @@ async def get_project_notes_route(project_id: int):
     uid = get_current_user_id()
     project = await get_project(uid, project_id)
     if project is None:
-        return jsonify({"error": "Project not found"}), 404
+        return not_found("Project")
 
     # type filter: "note", "task", or None (both)
     type_filter = request.args.get("type")