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/chat.py

@@ -6,6 +6,7 @@ import httpx
 from quart import Blueprint, Response, jsonify, request
 
 from fabledassistant.auth import login_required, get_current_user_id
+from fabledassistant.routes.utils import not_found
 from fabledassistant.config import Config
 from fabledassistant.services.chat import (
     add_message,
@@ -61,7 +62,7 @@ async def get_conversation_route(conv_id: int):
     uid = get_current_user_id()
     conv = await get_conversation(uid, conv_id)
     if conv is None:
-        return jsonify({"error": "Conversation not found"}), 404
+        return not_found("Conversation")
     result = conv.to_dict()
     note_ids = [m.context_note_id for m in conv.messages if m.context_note_id]
     note_map = await get_notes_by_ids(uid, note_ids) if note_ids else {}
@@ -80,7 +81,7 @@ async def delete_conversation_route(conv_id: int):
     uid = get_current_user_id()
     deleted = await delete_conversation(uid, conv_id)
     if not deleted:
-        return jsonify({"error": "Conversation not found"}), 404
+        return not_found("Conversation")
     return "", 204
 
 
@@ -95,7 +96,7 @@ async def update_conversation_route(conv_id: int):
         return jsonify({"error": "title or model is required"}), 400
     conv = await update_conversation(uid, conv_id, title=title, model=model)
     if conv is None:
-        return jsonify({"error": "Conversation not found"}), 404
+        return not_found("Conversation")
     return jsonify(conv.to_dict())
 
 
@@ -106,7 +107,7 @@ async def send_message_route(conv_id: int):
     uid = get_current_user_id()
     conv = await get_conversation(uid, conv_id)
     if conv is None:
-        return jsonify({"error": "Conversation not found"}), 404
+        return not_found("Conversation")
 
     data = await request.get_json()
     content = data.get("content", "").strip()
@@ -166,7 +167,7 @@ async def generation_stream_route(conv_id: int):
     uid = get_current_user_id()
     conv = await get_conversation(uid, conv_id)
     if conv is None:
-        return jsonify({"error": "Conversation not found"}), 404
+        return not_found("Conversation")
 
     buf = get_buffer(conv_id)
     if buf is None:
@@ -219,7 +220,7 @@ async def confirm_generation_route(conv_id: int):
     uid = get_current_user_id()
     conv = await get_conversation(uid, conv_id)
     if conv is None:
-        return jsonify({"error": "Conversation not found"}), 404
+        return not_found("Conversation")
 
     buf = get_buffer(conv_id)
     if buf is None or buf.state != GenerationState.RUNNING:
@@ -241,7 +242,7 @@ async def cancel_generation_route(conv_id: int):
     uid = get_current_user_id()
     conv = await get_conversation(uid, conv_id)
     if conv is None:
-        return jsonify({"error": "Conversation not found"}), 404
+        return not_found("Conversation")
 
     buf = get_buffer(conv_id)
     if buf is None or buf.state != GenerationState.RUNNING:
@@ -268,7 +269,7 @@ async def summarize_conversation_route(conv_id: int):
     uid = get_current_user_id()
     conv = await get_conversation(uid, conv_id)
     if conv is None:
-        return jsonify({"error": "Conversation not found"}), 404
+        return not_found("Conversation")
     model = await get_setting(uid, "default_model", Config.OLLAMA_MODEL) or Config.OLLAMA_MODEL
     try:
         note = await summarize_conversation_as_note(uid, conv_id, model)