feat: floor plan selection handles, dbl-click edit, bottom z-index

- Resize handles only render when the plan is selected (click to select,
  click outside to deselect); previously always shown when unlocked.
- Double-clicking an unlocked plan opens the canvas edit modal (via a
  floorMapEditNonce signal the Sidebar watches).
- Floor plan always sits at the bottom of the canvas (z-index -1), behind
  nodes and edges, locked or not.
- Remount the edit modal on every open (key bump) so it re-seeds from the
  current floor plan; fixes Save clobbering a canvas-side resize/move with
  stale modal dimensions.
- Drop the no-op history snapshot on floor-plan edits (floorMap isn't part
  of undo history).

frontend/src/stores/__tests__/canvasStore.test.ts

@@ -1337,5 +1337,12 @@ describe('canvasStore — custom style apply', () => {
       useCanvasStore.getState().updateFloorMap({ posX: 5 })
       expect(useCanvasStore.getState().floorMap).toBeNull()
     })
+
+    it('requestFloorMapEdit bumps the nonce each call', () => {
+      const start = useCanvasStore.getState().floorMapEditNonce
+      useCanvasStore.getState().requestFloorMapEdit()
+      useCanvasStore.getState().requestFloorMapEdit()
+      expect(useCanvasStore.getState().floorMapEditNonce).toBe(start + 2)
+    })
   })
 })

frontend/src/stores/canvasStore.ts

@@ -38,6 +38,10 @@ interface CanvasState {
   floorMap: FloorMapConfig | null
   setFloorMap: (config: FloorMapConfig | null) => void
   updateFloorMap: (patch: Partial<FloorMapConfig>) => void
+  // Bumped when the user double-clicks the floor plan on the canvas, asking the
+  // Sidebar to open the active canvas's edit modal (floor plan section).
+  floorMapEditNonce: number
+  requestFloorMapEdit: () => void
 
   // History
   past: HistoryEntry[]
@@ -103,6 +107,7 @@ export const useCanvasStore = create<CanvasState>((set) => ({
   scanEventTs: 0,
   serviceStatuses: {},
   floorMap: null,
+  floorMapEditNonce: 0,
   fitViewPending: false,
 
   past: [],
@@ -760,6 +765,8 @@ export const useCanvasStore = create<CanvasState>((set) => ({
       hasUnsavedChanges: true,
     })),
 
+  requestFloorMapEdit: () => set((s) => ({ floorMapEditNonce: s.floorMapEditNonce + 1 })),
+
   loadCanvas: (nodes, edges) => {
     // React Flow requires parents before children in the array
     const parents = nodes.filter((n) => !n.parentId)