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/db/models.py

@@ -22,6 +22,7 @@ class Design(Base):
     id: Mapped[str] = mapped_column(String, primary_key=True, default=_uuid)
     name: Mapped[str] = mapped_column(String, nullable=False)
     design_type: Mapped[str] = mapped_column(String, nullable=False, default="network")
+    icon: Mapped[str | None] = mapped_column(String, nullable=True, default="dashboard")
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=_now)
     updated_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=_now, onupdate=_now)