fix(notes): make drag-and-drop reorder and grouping work end to end

[sricharan12-hub]

Overview

Makes note drag-and-drop reordering and grouping/stacking work end to end.
Previously the feature was only partially wired: the drag events were never bound,
the API endpoints it called did not exist, and the frontend used a groupId field
that is not part of the Note schema — so reordering/grouping did nothing or 404'd.

Closes #80

Root causes fixed

1. Drag handlers never attached — NoteCard now sets draggable and binds
   onDragStart / onDragOver / onDrop / onDragEnd.
2. Missing / undefined props & state — NoteCard destructures all required
   props (activeDragId, setActiveDragId, allNotes, onMoveNote,
   onCombineNotes, onReorderNotes) and owns its own drag-indicator state,
   removing the undefined setDragIndicator / setChildDragIndicator references
   that would have thrown ReferenceError.
3. Endpoints didn't exist — added PATCH /notes/:id/reorder and
   POST /notes/group in routes + controllers.
4. Schema/field mismatch — standardized on parentId (already in the schema)
   everywhere; removed the non-existent groupId.

Changes

Backend

• PATCH /notes/:id/reorder — repositions a note among its siblings and, when a
  parentId is supplied, moves it into that container; renumbers position
  sequentially. Body: { targetId?, parentId? }.
• POST /notes/group — creates an isGroup: true note and sets both notes'
  parentId to it. Body: { sourceId, targetId, title }.
• Every query is scoped to req.user._id; ids validated with
  mongoose.Types.ObjectId.isValid.
• getAllNotes now sorts by position so a reorder persists across refresh.

Frontend

• NoteCard.jsx: full drag wiring, before/after/stack indicators, and grouped
  notes rendered stacked under their group card.
• HomePage.jsx: topLevelNotes filters on parentId; reorder/group handlers
  apply the authoritative note list returned by the endpoints.

Acceptance criteria

• Dragging a note onto another reorders it, and the new order persists after refresh.
• Dragging one note onto another can create a named group, and grouped notes render stacked.
• No 404s on /notes/:id/reorder or /notes/group; no ReferenceError during drag.
• Frontend and backend use a single consistent field name (parentId) for grouping.

Testing

• eslint passes on HomePage.jsx and NoteCard.jsx; node --check passes on the
  backend controller and routes.
• Simulated the reorder insertion/renumbering algorithm across representative cases
  (top-level reorder, move into group, append to group, reorder within group) — all
  produce correct sequential position values and parentId updates.

Note: this repo has no automated test suite or local DB wired up, so backend
handlers were verified via static checks and an algorithm simulation rather than a
live integration run.

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📥 Commits

Reviewing files that changed from the base of the PR and between 87148ba and 6ccea76.

📒 Files selected for processing (4)

• backend/src/controllers/notesController.js
• backend/src/routes/notesRoutes.js
• frontend/src/components/NoteCard.jsx
• frontend/src/pages/HomePage.jsx

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• Create PR with unit tests

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