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/api/routes/canvas.py

@@ -25,6 +25,7 @@ async def load_canvas(db: AsyncSession = Depends(get_db), _: str = Depends(get_c
         nodes=[NodeResponse.model_validate(n) for n in nodes],
         edges=[EdgeResponse.model_validate(e) for e in edges],
         viewport=viewport,
+        custom_style=state.custom_style if state else None,
     )
 
 
@@ -67,13 +68,14 @@ async def save_canvas(
         else:
             db.add(Edge(**edge_data.model_dump()))
 
-    # Upsert viewport
+    # Upsert viewport + custom style
     state = await db.get(CanvasState, 1)
     if state:
         state.viewport = body.viewport
+        state.custom_style = body.custom_style
         state.saved_at = datetime.now(timezone.utc)
     else:
-        db.add(CanvasState(id=1, viewport=body.viewport))
+        db.add(CanvasState(id=1, viewport=body.viewport, custom_style=body.custom_style))
 
     await db.commit()
     return {"saved": True}