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/tasks.py

@@ -1,9 +1,8 @@
-from datetime import date
-
 from quart import Blueprint, jsonify, request
 
 from fabledassistant.auth import login_required, get_current_user_id
 from fabledassistant.models.note import TaskPriority, TaskStatus
+from fabledassistant.routes.utils import not_found, parse_iso_date
 from fabledassistant.services.notes import (
     create_note,
     delete_note,
@@ -28,13 +27,12 @@ async def list_tasks_route():
     limit = min(request.args.get("limit", 50, type=int), 500)
     offset = request.args.get("offset", 0, type=int)
 
-    due_before_str = request.args.get("due_before")
-    due_after_str = request.args.get("due_after")
-    try:
-        due_before = date.fromisoformat(due_before_str) if due_before_str else None
-        due_after = date.fromisoformat(due_after_str) if due_after_str else None
-    except ValueError:
-        return jsonify({"error": "Invalid date format. Use YYYY-MM-DD."}), 400
+    due_before = parse_iso_date(request.args.get("due_before"), "due_before")
+    if isinstance(due_before, tuple):
+        return due_before
+    due_after = parse_iso_date(request.args.get("due_after"), "due_after")
+    if isinstance(due_after, tuple):
+        return due_after
 
     tasks, total = await list_notes(
         uid,
@@ -61,12 +59,9 @@ async def create_task_route():
     body = data.get("body", "") or data.get("description", "")
     tags = data.get("tags", [])
 
-    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
 
     status = TaskStatus(data["status"]).value if "status" in data else TaskStatus.todo.value
     priority = (
@@ -94,7 +89,7 @@ async def get_task_route(task_id: int):
     uid = get_current_user_id()
     task = await get_note(uid, task_id)
     if task is None:
-        return jsonify({"error": "Task not found"}), 404
+        return not_found("Task")
     data = task.to_dict()
     if task.parent_id:
         parent = await get_note(uid, task.parent_id)
@@ -120,10 +115,10 @@ async def update_task_route(task_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
 
@@ -136,7 +131,7 @@ async def update_task_route(task_id: int):
 
     task = await update_note(uid, task_id, **fields)
     if task is None:
-        return jsonify({"error": "Task not found"}), 404
+        return not_found("Task")
     return jsonify(task.to_dict())
 
 
@@ -156,7 +151,7 @@ async def patch_task_status(task_id: int):
 
     task = await update_note(uid, task_id, status=status_val)
     if task is None:
-        return jsonify({"error": "Task not found"}), 404
+        return not_found("Task")
     return jsonify(task.to_dict())
 
 
@@ -166,5 +161,5 @@ async def delete_task_route(task_id: int):
     uid = get_current_user_id()
     deleted = await delete_note(uid, task_id)
     if not deleted:
-        return jsonify({"error": "Task not found"}), 404
+        return not_found("Task")
     return "", 204