feat: manage canvases with custom name and icon

Make designs (canvases) fully user-manageable: create with a chosen name
and icon, rename, change icon, and delete. Replaces the hardcoded
"New Electrical Design" button with a generic "New Canvas" flow.

- Add Design.icon column + migration that backfills legacy rows
  (electrical -> zap, others -> dashboard)
- DesignModal: name input + curated lucide icon picker (create + edit)
- Sidebar switcher gains per-canvas edit/delete; delete guards the last
  canvas and confirms
- designStore: addDesign/updateDesign/removeDesign with active reassignment
- Fix data loss on design switch: abort load when the save fails and keep
  unsaved edits; skip the save-old step when the previous canvas was deleted
- designsApi create/update carry icon; design_type kept for back-compat

Tests: backend design CRUD (icon + cascade + last-canvas guard), designStore
actions, designIcons resolver, DesignModal create/edit/validation.

ha-relevant: yes

backend/app/api/routes/designs.py

@@ -25,7 +25,7 @@ async def create_design(
     db: AsyncSession = Depends(get_db),
     _: str = Depends(get_current_user),
 ) -> DesignResponse:
-    design = Design(name=body.name, design_type=body.design_type)
+    design = Design(name=body.name, design_type=body.design_type, icon=body.icon)
     db.add(design)
     await db.flush()
     # Create empty canvas state for the new design
@@ -47,6 +47,8 @@ async def update_design(
         raise HTTPException(404, "Design not found")
     if body.name is not None:
         design.name = body.name
+    if body.icon is not None:
+        design.icon = body.icon
     await db.commit()
     await db.refresh(design)
     return DesignResponse.model_validate(design)