feat: floor plan viewport rendering, per-canvas config, server media upload

- Render floor plan inside React Flow ViewportPortal so it pans/zooms with
  nodes (was screen-fixed, desynced on pan/zoom); zoom-stable resize handles.
- Move floor plan config from the left panel into the canvas (design) edit
  modal; attach per-design and fix cross-design bleed on load.
- Store images via a new generic backend media endpoint (POST/GET/DELETE
  /api/v1/media) on disk under <data_dir>/uploads, not base64 in the canvas.
- Disable floor plans in standalone mode (no backend to upload/serve); drop
  base64 localStorage persistence. See ADR-001 in CLAUDE.md.
- Tests: backend media route, DesignModal floor plan + upload, store floorMap.

ha-relevant: maybe

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

@@ -1305,4 +1305,37 @@ describe('canvasStore — custom style apply', () => {
     expect(n.width).toBe(300)
     expect(n.height).toBe(100)
   })
+
+  describe('floorMap', () => {
+    const fm = {
+      imageData: 'data:image/png;base64,abc',
+      posX: 10, posY: 20, width: 800, height: 600,
+      opacity: 0.8, locked: false, enabled: true,
+    }
+
+    beforeEach(() => useCanvasStore.setState({ floorMap: null, hasUnsavedChanges: false }))
+
+    it('setFloorMap sets and marks unsaved', () => {
+      useCanvasStore.getState().setFloorMap(fm)
+      expect(useCanvasStore.getState().floorMap).toEqual(fm)
+      expect(useCanvasStore.getState().hasUnsavedChanges).toBe(true)
+    })
+
+    it('setFloorMap(null) clears the plan — prevents cross-design bleed on load', () => {
+      useCanvasStore.setState({ floorMap: fm })
+      useCanvasStore.getState().setFloorMap(null)
+      expect(useCanvasStore.getState().floorMap).toBeNull()
+    })
+
+    it('updateFloorMap patches an existing plan (position preserved)', () => {
+      useCanvasStore.setState({ floorMap: fm })
+      useCanvasStore.getState().updateFloorMap({ posX: 99, opacity: 0.5 })
+      expect(useCanvasStore.getState().floorMap).toEqual({ ...fm, posX: 99, opacity: 0.5 })
+    })
+
+    it('updateFloorMap is a no-op when no plan exists', () => {
+      useCanvasStore.getState().updateFloorMap({ posX: 5 })
+      expect(useCanvasStore.getState().floorMap).toBeNull()
+    })
+  })
 })