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/models/__init__.py

@@ -16,6 +16,9 @@ class Base(DeclarativeBase):
     pass
 
 
+from fabledassistant.models.base import CreatedAtMixin, TimestampMixin  # noqa: E402, F401
+
+
 from fabledassistant.models.note import Note, TaskPriority, TaskStatus  # noqa: E402, F401
 from fabledassistant.models.conversation import Conversation, Message  # noqa: E402, F401
 from fabledassistant.models.setting import Setting  # noqa: E402, F401