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)

backend/app/db/models.py

@@ -78,6 +78,7 @@ class CanvasState(Base):
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True, default=1)
     viewport: Mapped[dict[str, Any]] = mapped_column(JSON, default=dict)
+    custom_style: Mapped[dict[str, Any] | None] = mapped_column(JSON, nullable=True)
     saved_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=_now)