Tab icon size is hard-coded to 27x27pt with no way to override it

### Summary

Tab icon size is hard-coded to 27×27pt with no way to override it, which makes it impossible to give a single tab item a larger icon.

### Where

Two independent call sites, both fixed:

- [`ios/TabViewProvider.swift:60`](https://github.com/callstack/react-native-bottom-tabs/blob/main/packages/react-native-bottom-tabs/ios/TabViewProvider.swift#L60) — `private var iconSize = CGSize(width: 27, height: 27)`, applied to remotely-loaded icons via `image.resizeImageTo(size: iconSize)`.
- [`ios/TabViewImpl.swift:350`](https://github.com/callstack/react-native-bottom-tabs/blob/main/packages/react-native-bottom-tabs/ios/TabViewImpl.swift#L350) — `let iconSize = CGSize(width: 27, height: 27)` inside `makeTabBarItemImage`, used on the `experimental_bakedTintColors` path.

Because remote images are resized to `iconSize`, supplying a larger source image has no effect.

### Use case

We show the signed-in user's avatar as the Profile tab's icon (a snapshot of our own circular avatar component, handed over as a `file://` PNG). Design wants that avatar rendered noticeably larger than the neighbouring glyph icons, which is a common pattern for a "you" tab — the avatar is a photo of a person rather than a symbol, so it reads better at a larger size, especially with the label hidden via `tabBarLabel: ''`.

At 27pt it is legible but visibly smaller than intended, and there is currently no prop, per-screen option, or `imageInsets`-style escape hatch to change it.

### Proposal

Expose the icon size, ideally per screen so one item can differ from the rest:

```tsx
({ uri: avatarFileUri }),
tabBarIconSize: 34, // new
}}
/>
```

A navigator-level default (`iconSize` on `Tab.Navigator`) with a per-screen override would also work for us. Happy to open a PR if you'd like it in this shape — wanted to check the preferred API first, and whether per-screen sizing raises layout concerns on Android's `BottomNavigationView` that aren't obvious from the iOS side.

### Environment

- `react-native-bottom-tabs` 1.4.0 / `@bottom-tabs/react-navigation` 1.4.0
- React Native 0.85.3, Expo SDK 56, New Architecture, React 19.2.3
- iOS 26.5 simulator (iPhone 17 Pro), Xcode 26.6

Bắt đầu với React Native tab options API và hai vị trí gọi iOS trong ios/TabViewProvider.swift:60 và ios/TabViewImpl.swift:350. Kiểm tra cách một navigator-level default và một per-screen override sẽ được truyền đến cả hai đường dẫn kích thước cố định, bao gồm vấn đề layout của Android được đề cập trong proposal. Được xem là hoàn tất khi API được chọn cho phép một tab icon sử dụng kích thước khác mà không làm hỏng hành vi mặc định.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.