feat: add custom style theme with per-type node/edge style editor

- New 'custom' ThemeId with dedicated ThemeCard + pencil edit button
- CustomStyleModal: two-column editor for all node types (excl. groupRect/group) and edge types
  - Per node: border, background, icon color + opacity, default size (w/h)
  - Per edge: color + opacity, path style, animation
  - 'Apply to existing [Type]' per type, 'Apply All to Canvas' footer action
- canvasStore: applyTypeNodeStyle, applyTypeEdgeStyle, applyAllCustomStyles actions
- themeStore: customStyle state + setCustomStyle action
- Backend: custom_style JSON column on canvas_state (ALTER TABLE migration), saved/loaded with canvas
- App.tsx: custom_style included in save payload, restored on load (API + standalone)
- Tests: +8 frontend (themeStore + canvasStore), +3 backend (canvas API)

frontend/src/stores/themeStore.ts

@@ -1,12 +1,17 @@
 import { create } from 'zustand'
 import type { ThemeId } from '@/utils/themes'
+import type { CustomStyleDef } from '@/types'
 
 interface ThemeState {
   activeTheme: ThemeId
   setTheme: (id: ThemeId) => void
+  customStyle: CustomStyleDef
+  setCustomStyle: (def: CustomStyleDef) => void
 }
 
 export const useThemeStore = create<ThemeState>((set) => ({
   activeTheme: 'default',
   setTheme: (id) => set({ activeTheme: id }),
+  customStyle: { nodes: {}, edges: {} },
+  setCustomStyle: (def) => set({ customStyle: def }),
 }))