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/note.py

@@ -1,11 +1,12 @@
 import enum
-from datetime import date, datetime, timezone
+from datetime import date
 
-from sqlalchemy import Date, DateTime, ForeignKey, Index, Integer, Text
+from sqlalchemy import Date, ForeignKey, Index, Integer, Text
 from sqlalchemy.dialects.postgresql import ARRAY
 from sqlalchemy.orm import Mapped, mapped_column
 
 from fabledassistant.models import Base
+from fabledassistant.models.base import TimestampMixin
 
 
 class TaskStatus(str, enum.Enum):
@@ -21,7 +22,7 @@ class TaskPriority(str, enum.Enum):
     high = "high"
 
 
-class Note(Base):
+class Note(Base, TimestampMixin):
     __tablename__ = "notes"
 
     id: Mapped[int] = mapped_column(primary_key=True)
@@ -43,14 +44,6 @@ class Note(Base):
     status: Mapped[str | None] = mapped_column(Text, nullable=True)
     priority: Mapped[str | None] = mapped_column(Text, nullable=True)
     due_date: Mapped[date | None] = mapped_column(Date, nullable=True)
-    created_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
-    )
-    updated_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True),
-        default=lambda: datetime.now(timezone.utc),
-        onupdate=lambda: datetime.now(timezone.utc),
-    )
 
     __table_args__ = (
         Index("ix_notes_tags", "tags", postgresql_using="gin"),