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/notes.py

@@ -1,13 +1,13 @@
 import asyncio
 import logging
 import re
-from datetime import date
 
 from fabledassistant.services.embeddings import upsert_note_embedding
 
 from quart import Blueprint, Response, jsonify, request
 
 from fabledassistant.auth import login_required, get_current_user_id
+from fabledassistant.routes.utils import not_found, parse_iso_date
 from fabledassistant.config import Config
 from fabledassistant.services.assist import build_assist_messages
 from fabledassistant.services.generation_buffer import (
@@ -87,12 +87,9 @@ async def create_note_route():
     # Optional task fields
     status = data.get("status")
     priority = data.get("priority")
-    due_date = None
-    if data.get("due_date"):
-        try:
-            due_date = date.fromisoformat(data["due_date"])
-        except ValueError:
-            return jsonify({"error": "Invalid date format. Use YYYY-MM-DD."}), 400
+    due_date = parse_iso_date(data.get("due_date"), "due_date")
+    if isinstance(due_date, tuple):
+        return due_date
 
     note = await create_note(
         uid,
@@ -144,7 +141,7 @@ async def append_tag_route(note_id: int):
 
     note = await get_note(uid, note_id)
     if note is None:
-        return jsonify({"error": "Note not found"}), 404
+        return not_found("Note")
 
     existing = list(note.tags or [])
     if tag not in existing:
@@ -162,7 +159,7 @@ async def get_note_by_title_route():
         return jsonify({"error": "title parameter is required"}), 400
     note = await get_note_by_title(uid, title)
     if note is None:
-        return jsonify({"error": "Note not found"}), 404
+        return not_found("Note")
     return jsonify(note.to_dict())
 
 
@@ -184,7 +181,7 @@ async def get_note_route(note_id: int):
     uid = get_current_user_id()
     note = await get_note(uid, note_id)
     if note is None:
-        return jsonify({"error": "Note not found"}), 404
+        return not_found("Note")
     return jsonify(note.to_dict())
 
 
@@ -200,10 +197,10 @@ async def update_note_route(note_id: int):
 
     if "due_date" in data:
         if data["due_date"]:
-            try:
-                fields["due_date"] = date.fromisoformat(data["due_date"])
-            except ValueError:
-                return jsonify({"error": "Invalid date format. Use YYYY-MM-DD."}), 400
+            result = parse_iso_date(data["due_date"], "due_date")
+            if isinstance(result, tuple):
+                return result
+            fields["due_date"] = result
         else:
             fields["due_date"] = None
 
@@ -211,7 +208,7 @@ async def update_note_route(note_id: int):
         fields["tags"] = data["tags"]
     note = await update_note(uid, note_id, **fields)
     if note is None:
-        return jsonify({"error": "Note not found"}), 404
+        return not_found("Note")
     text = f"{note.title}\n{note.body}".strip() if note.body else (note.title or "")
     if text:
         asyncio.create_task(upsert_note_embedding(note.id, uid, text))
@@ -228,15 +225,15 @@ async def patch_note_route(note_id: int):
         if key in data:
             fields[key] = data[key]
     if "due_date" in data:
-        try:
-            fields["due_date"] = date.fromisoformat(data["due_date"]) if data["due_date"] else None
-        except (ValueError, TypeError):
-            return jsonify({"error": "Invalid due_date format, expected YYYY-MM-DD"}), 400
+        result = parse_iso_date(data.get("due_date"), "due_date")
+        if isinstance(result, tuple):
+            return result
+        fields["due_date"] = result
     if "tags" in data:
         fields["tags"] = data["tags"]
     note = await update_note(uid, note_id, **fields)
     if note is None:
-        return jsonify({"error": "Note not found"}), 404
+        return not_found("Note")
     return jsonify(note.to_dict())
 
 
@@ -246,7 +243,7 @@ async def delete_note_route(note_id: int):
     uid = get_current_user_id()
     deleted = await delete_note(uid, note_id)
     if not deleted:
-        return jsonify({"error": "Note not found"}), 404
+        return not_found("Note")
     return "", 204
 
 
@@ -474,7 +471,7 @@ async def upsert_draft_route(note_id: int):
     # Verify note ownership
     note = await get_note(uid, note_id)
     if note is None:
-        return jsonify({"error": "Note not found"}), 404
+        return not_found("Note")
     data = await request.get_json()
     draft = await upsert_draft(
         user_id=uid,
@@ -511,5 +508,5 @@ async def get_version_route(note_id: int, version_id: int):
     uid = get_current_user_id()
     version = await get_version(uid, note_id, version_id)
     if version is None:
-        return jsonify({"error": "Version not found"}), 404
+        return not_found("Version")
     return jsonify(version.to_dict(include_body=True))