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

frontend/src/utils/__tests__/designIcons.test.ts

@@ -0,0 +1,27 @@
+import { describe, it, expect } from 'vitest'
+import { DESIGN_ICONS, DEFAULT_DESIGN_ICON, resolveDesignIcon } from '@/utils/designIcons'
+
+describe('designIcons', () => {
+  it('exposes a non-empty, unique-keyed icon set', () => {
+    expect(DESIGN_ICONS.length).toBeGreaterThan(0)
+    const keys = DESIGN_ICONS.map((e) => e.key)
+    expect(new Set(keys).size).toBe(keys.length)
+  })
+
+  it('default icon key is part of the set', () => {
+    expect(DESIGN_ICONS.some((e) => e.key === DEFAULT_DESIGN_ICON)).toBe(true)
+  })
+
+  it('resolveDesignIcon returns the matching component for a known key', () => {
+    const entry = DESIGN_ICONS.find((e) => e.key === 'zap')!
+    expect(resolveDesignIcon('zap')).toBe(entry.icon)
+  })
+
+  it('resolveDesignIcon falls back to a component for unknown/empty keys', () => {
+    const fallback = resolveDesignIcon(undefined)
+    expect(typeof fallback).toBe('object')
+    expect(resolveDesignIcon('does-not-exist')).toBe(fallback)
+    expect(resolveDesignIcon(null)).toBe(fallback)
+    expect(resolveDesignIcon('')).toBe(fallback)
+  })
+})