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/utils/colorUtils.ts

@@ -27,3 +27,17 @@ export function rgbaToHex8(hex6: string, alpha: number): string {
   const alphaHex = alphaByte.toString(16).padStart(2, '0')
   return `${hex6}${alphaHex}`
 }
+
+/**
+ * Combine a hex color and opacity (0–1) into a CSS rgba() string.
+ * Returns the plain hex when opacity is 1.
+ */
+export function applyOpacity(hex: string, opacity: number): string {
+  if (opacity >= 1) return hex
+  let h = hex.replace('#', '')
+  if (h.length === 3) h = h[0]+h[0]+h[1]+h[1]+h[2]+h[2]
+  const r = parseInt(h.slice(0, 2), 16)
+  const g = parseInt(h.slice(2, 4), 16)
+  const b = parseInt(h.slice(4, 6), 16)
+  return `rgba(${r}, ${g}, ${b}, ${Math.round(opacity * 100) / 100})`
+}