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/types/index.ts

@@ -150,3 +150,26 @@ export const EDGE_TYPE_LABELS: Record<EdgeType, string> = {
   virtual: 'Virtual',
   cluster: 'Cluster',
 }
+
+export interface NodeTypeStyle {
+  borderColor: string
+  borderOpacity: number
+  bgColor: string
+  bgOpacity: number
+  iconColor: string
+  iconOpacity: number
+  width: number
+  height: number
+}
+
+export interface EdgeTypeStyle {
+  color: string
+  opacity: number
+  pathStyle: EdgePathStyle
+  animated: 'none' | 'snake' | 'flow' | 'basic'
+}
+
+export interface CustomStyleDef {
+  nodes: Partial<Record<NodeType, NodeTypeStyle>>
+  edges: Partial<Record<EdgeType, EdgeTypeStyle>>
+}