M2 drag-reorder: notes.position + reorder API + native DnD

- Migration 0008: notes.position (int). Board orders pinned -> position ->
  updated_at; new notes created at top (max position + 1). POST /api/notes/reorder
  assigns positions from the given order (owner-scoped).
- notes store: position on Note, position-aware sort, optimistic reorder().
- NoteCard reorderable (native HTML5 draggable + dragstart/drop); BoardView moves
  the dragged note before the drop target and persists.

Note: drag on a CSS-columns masonry has imperfect during-drag visuals (columns
reflow); order persists correctly. Candidate for a polish pass / layout tweak.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/stores/notes.ts

@@ -30,6 +30,7 @@ export interface Note {
   body: string;
   color: NoteColor;
   kind: NoteKind;
+  position: number;
   pinned: boolean;
   archived: boolean;
   trashed: boolean;
@@ -49,6 +50,7 @@ export const useNotesStore = defineStore("notes", () => {
   function sortItems(): void {
     items.value.sort((a, b) => {
       if (a.pinned !== b.pinned) return a.pinned ? -1 : 1;
+      if (a.position !== b.position) return b.position - a.position;
       return (b.updated_at ?? "").localeCompare(a.updated_at ?? "");
     });
   }
@@ -139,6 +141,18 @@ export const useNotesStore = defineStore("notes", () => {
     reconcile(await api.del<Note>(`/api/notes/${id}/attachments/${attId}`));
   }
 
+  async function reorder(orderedIds: string[]): Promise<void> {
+    // Optimistically assign positions matching the backend (total - index), sort,
+    // then persist.
+    const total = orderedIds.length;
+    orderedIds.forEach((id, index) => {
+      const n = items.value.find((x) => x.id === id);
+      if (n) n.position = total - index;
+    });
+    sortItems();
+    await api.post("/api/notes/reorder", { ids: orderedIds });
+  }
+
   async function trash(id: string): Promise<void> {
     reconcile(await api.post<Note>(`/api/notes/${id}/trash`));
   }
@@ -171,6 +185,7 @@ export const useNotesStore = defineStore("notes", () => {
     deleteItem,
     uploadAttachment,
     deleteAttachment,
+    reorder,
     trash,
     restore,
     deleteForever,