Add drag-and-drop reordering for note tabs, and a user-facing guide

Tabs can now be dragged to reorder within a group and to move between
sections in one gesture: dropping on a tab's left/right half inserts
before/after it and adopts that tab's section, while a dedicated zone
past the last group ungroups and appends at the end. A blue indicator
line shows the drop position while dragging.

core/notes.reorder() takes the full new page-id order computed
client-side (NotePanel.vue has the reactive list already) rather than
recomputing positions server-side, validates it against the current
page-id set, sets the moved page's section, then renumbers position.
POST /api/notes/reorder exposes it.

Also adds docs/notes-guide.md, an end-user walkthrough of the shared
notes panel (pages, sections, drag-and-drop, AI interaction) — the
existing docs were all developer-facing specs with no operating
instructions for the feature itself.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/app/api/notes.py

@@ -4,7 +4,7 @@ from fastapi import APIRouter, Depends, HTTPException
 from pydantic import BaseModel
 from sqlalchemy.orm import Session as DBSession
 
-from app.core.notes import next_position, rename_section, set_section
+from app.core.notes import next_position, reorder, rename_section, set_section
 from app.db.database import get_db
 from app.db.models import NotePage
 
@@ -40,6 +40,12 @@ class SectionRenameRequest(BaseModel):
     new_name: str | None = None
 
 
+class ReorderRequest(BaseModel):
+    page_id: int
+    section: str | None = None
+    order: list[int]
+
+
 @router.get("", response_model=list[NotePageResponse])
 def list_pages(db: DBSession = Depends(get_db)):
     return db.query(NotePage).order_by(NotePage.position, NotePage.id).all()
@@ -64,6 +70,15 @@ def rename_section_endpoint(req: SectionRenameRequest, db: DBSession = Depends(g
     return {"renamed": count}
 
 
+@router.post("/reorder")
+def reorder_pages(req: ReorderRequest, db: DBSession = Depends(get_db)):
+    """タブのドラッグ&ドロップによる並べ替え。order は全ページIDを新しい順序で並べたもの。"""
+    ok = reorder(db, req.page_id, req.section, req.order)
+    if not ok:
+        raise HTTPException(status_code=400, detail="order must contain exactly the current page ids")
+    return {"reordered": len(req.order)}
+
+
 @router.patch("/{page_id}", response_model=NotePageResponse)
 def update_page(page_id: int, req: NotePageUpdate, db: DBSession = Depends(get_db)):
     page = db.get(NotePage, page_id)