feat: create a new canvas by copying an existing one

Add a 'Copy from existing' option to the New Canvas modal. It lists every
canvas with node/group/text counts; picking one deep-copies its nodes, edges,
parent/child links and canvas state (viewport, custom style, floor plan) into
a fresh design.

Backend: POST /designs/{source_id}/copy remaps node ids, re-points edges and
parent links, and clones canvas state; GET /designs now returns per-design
counts for the picker. Standalone mode clones the localStorage canvas.

Closes #216

ha-relevant: maybe

backend/app/api/routes/designs.py

@@ -1,14 +1,21 @@
+import uuid
+
 from fastapi import APIRouter, Depends, HTTPException
-from sqlalchemy import select
+from sqlalchemy import func, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.api.deps import get_current_user
 from app.db.database import get_db
 from app.db.models import CanvasState, Design, Edge, Node
-from app.schemas.designs import DesignCreate, DesignResponse, DesignUpdate
+from app.schemas.designs import DesignCopy, DesignCreate, DesignResponse, DesignUpdate
 
 router = APIRouter()
 
+# Node.type values that are canvas annotations rather than real devices. Kept in
+# sync with the frontend (Sidebar counts, canvasSerializer types).
+_GROUP_TYPE = "groupRect"
+_TEXT_TYPE = "text"
+
 
 @router.get("", response_model=list[DesignResponse])
 async def list_designs(
@@ -16,7 +23,31 @@ async def list_designs(
     _: str = Depends(get_current_user),
 ) -> list[DesignResponse]:
     designs = (await db.execute(select(Design).order_by(Design.created_at))).scalars().all()
-    return [DesignResponse.model_validate(d) for d in designs]
+    # One grouped query for all designs → node/group/text counts per design.
+    rows = (
+        await db.execute(select(Node.design_id, Node.type, func.count()).group_by(Node.design_id, Node.type))
+    ).all()
+    counts: dict[str, dict[str, int]] = {}
+    for design_id, node_type, count in rows:
+        if design_id is None:
+            continue
+        bucket = counts.setdefault(design_id, {"node": 0, "group": 0, "text": 0})
+        if node_type == _GROUP_TYPE:
+            bucket["group"] += count
+        elif node_type == _TEXT_TYPE:
+            bucket["text"] += count
+        else:
+            bucket["node"] += count
+
+    result = []
+    for d in designs:
+        resp = DesignResponse.model_validate(d)
+        c = counts.get(d.id, {"node": 0, "group": 0, "text": 0})
+        resp.node_count = c["node"]
+        resp.group_count = c["group"]
+        resp.text_count = c["text"]
+        result.append(resp)
+    return result
 
 
 @router.post("", response_model=DesignResponse, status_code=201)
@@ -35,6 +66,73 @@ async def create_design(
     return DesignResponse.model_validate(design)
 
 
+@router.post("/{source_id}/copy", response_model=DesignResponse, status_code=201)
+async def copy_design(
+    source_id: str,
+    body: DesignCopy,
+    db: AsyncSession = Depends(get_db),
+    _: str = Depends(get_current_user),
+) -> DesignResponse:
+    """Create a new design that deep-copies the source's nodes, edges and canvas state."""
+    source = await db.get(Design, source_id)
+    if not source:
+        raise HTTPException(404, "Source design not found")
+
+    new_design = Design(name=body.name, icon=body.icon, design_type=source.design_type)
+    db.add(new_design)
+    await db.flush()
+
+    src_nodes = (await db.execute(select(Node).where(Node.design_id == source_id))).scalars().all()
+    src_edges = (await db.execute(select(Edge).where(Edge.design_id == source_id))).scalars().all()
+
+    # New id per source node so edges and parent links can be re-pointed at the copy.
+    id_map = {n.id: str(uuid.uuid4()) for n in src_nodes}
+
+    # Columns we set explicitly or let the DB default — never copy verbatim.
+    node_skip = {"id", "design_id", "parent_id", "created_at", "updated_at"}
+    for n in src_nodes:
+        cols = {c.name: getattr(n, c.name) for c in Node.__table__.columns if c.name not in node_skip}
+        db.add(Node(id=id_map[n.id], design_id=new_design.id, parent_id=None, **cols))
+    await db.flush()  # nodes must exist before we wire self-referential parent_id
+
+    # Second pass: re-point parent links inside the copy.
+    for n in src_nodes:
+        if n.parent_id and n.parent_id in id_map:
+            child = await db.get(Node, id_map[n.id])
+            if child:
+                child.parent_id = id_map[n.parent_id]
+
+    edge_skip = {"id", "design_id", "source", "target", "created_at"}
+    for e in src_edges:
+        # Skip edges whose endpoints aren't part of this design (dangling FKs).
+        if e.source not in id_map or e.target not in id_map:
+            continue
+        cols = {c.name: getattr(e, c.name) for c in Edge.__table__.columns if c.name not in edge_skip}
+        db.add(
+            Edge(
+                id=str(uuid.uuid4()),
+                design_id=new_design.id,
+                source=id_map[e.source],
+                target=id_map[e.target],
+                **cols,
+            )
+        )
+
+    # Copy canvas state (viewport, custom style, and the floor plan carried in viewport).
+    src_state = await db.get(CanvasState, source_id)
+    db.add(
+        CanvasState(
+            design_id=new_design.id,
+            viewport=src_state.viewport if src_state else {},
+            custom_style=src_state.custom_style if src_state else None,
+        )
+    )
+
+    await db.commit()
+    await db.refresh(new_design)
+    return DesignResponse.model_validate(new_design)
+
+
 @router.put("/{design_id}", response_model=DesignResponse)
 async def update_design(
     design_id: str,