fix(storybook): manager.tsx imports relatively (esbuild manager has no aliases)

The prior change aliased manager.tsx to @core, but Storybook's manager bundle
(esbuild) doesn't resolve aliases — only the preview/Vite bundle does — so the
storybook build failed. Revert manager.tsx to a relative import and scope a
one-file no-restricted-imports exemption to .storybook/manager.tsx (the only
file that genuinely can't use an alias). preview.tsx keeps @public (Vite-built).

Verified: eslint clean + storybook build succeeds.

frontend/.storybook/manager.tsx

@@ -11,7 +11,7 @@ import { IconButton, WithTooltip } from "storybook/internal/components";
 import {
   THEME_ACCENT_PRESETS as PRESETS,
   DEFAULT_ACCENT_COLOR,
-} from "@core/constants/theme";
+} from "../editor/src/core/constants/theme";
 
 function AccentPicker() {
   const [globals, updateGlobals] = useGlobals();

frontend/eslint.config.mjs

@@ -324,4 +324,13 @@ export default defineConfig(
       },
     },
   },
+  // Storybook's MANAGER is bundled by esbuild with no alias resolution (only the
+  // preview/Vite bundle resolves @app/@core/etc). manager.tsx must therefore
+  // import editor code relatively — an alias would fail the manager build.
+  {
+    files: [".storybook/manager.tsx"],
+    rules: {
+      "no-restricted-imports": "off",
+    },
+  },
 );