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/conversation.py

@@ -1,14 +1,13 @@
-from datetime import datetime, timezone
-
-from sqlalchemy import DateTime, ForeignKey, Index, Integer, Text
+from sqlalchemy import ForeignKey, Index, Integer, Text
 from sqlalchemy import inspect
 from sqlalchemy.dialects.postgresql import JSONB
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from fabledassistant.models import Base
+from fabledassistant.models.base import CreatedAtMixin, TimestampMixin
 
 
-class Conversation(Base):
+class Conversation(Base, TimestampMixin):
     __tablename__ = "conversations"
 
     id: Mapped[int] = mapped_column(primary_key=True)
@@ -17,14 +16,6 @@ class Conversation(Base):
     )
     title: Mapped[str] = mapped_column(Text, default="")
     model: Mapped[str] = mapped_column(Text, default="")
-    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),
-    )
 
     messages: Mapped[list["Message"]] = relationship(
         back_populates="conversation",
@@ -53,7 +44,7 @@ class Conversation(Base):
         }
 
 
-class Message(Base):
+class Message(Base, CreatedAtMixin):
     __tablename__ = "messages"
 
     id: Mapped[int] = mapped_column(primary_key=True)
@@ -67,9 +58,6 @@ class Message(Base):
         Integer, ForeignKey("notes.id", ondelete="SET NULL"), nullable=True
     )
     tool_calls: Mapped[list | None] = mapped_column(JSONB, nullable=True)
-    created_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
-    )
 
     conversation: Mapped["Conversation"] = relationship(back_populates="messages")