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/user.py

@@ -1,12 +1,11 @@
-from datetime import datetime, timezone
-
-from sqlalchemy import DateTime, Index, Text
+from sqlalchemy import Index, Text
 from sqlalchemy.orm import Mapped, mapped_column
 
 from fabledassistant.models import Base
+from fabledassistant.models.base import CreatedAtMixin
 
 
-class User(Base):
+class User(Base, CreatedAtMixin):
     __tablename__ = "users"
 
     id: Mapped[int] = mapped_column(primary_key=True)
@@ -15,9 +14,6 @@ class User(Base):
     password_hash: Mapped[str | None] = mapped_column(Text, nullable=True)
     oauth_sub: Mapped[str | None] = mapped_column(Text, unique=True, nullable=True)
     role: Mapped[str] = mapped_column(Text, nullable=False, default="user")
-    created_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
-    )
 
     __table_args__ = (
         Index("ix_users_username", "username"),