chore: refactor imports and remove unused code

- Add `core-js` package as a dependency in `package.json`
- Remove `ress.css`, `global.scss`, and `overrides.scss` imports from `main.ts` and `root.tsx`
- Add `polyfill.ts` file with imports and function calls
- Remove `MantineProvider` and `useColorScheme` imports from `App.tsx`
- Add `mantineTheme` import in `App.tsx`
- Remove `enableMapSet()` and `setAutoFreeze(true)` calls from `main.ts` and `root.tsx`
- Remove unused imports in `App.tsx` and `root.tsx`
- Remove unused variables in `App.tsx` and `root.tsx`
- Move `octokit` initialization to a separate file

Signed-off-by: Eva1ent <let@ik.me>

package.json

@@ -23,6 +23,7 @@
         "@swan-io/chicane": "1.4.1",
         "@tauri-apps/api": "1.4.0",
         "@total-typescript/ts-reset": "0.4.2",
+        "core-js": "3.31.1",
         "immer": "10.0.2",
         "jotai": "2.2.2",
         "jotai-immer": "0.2.0",