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

React Native tab options APIと、ios/TabViewProvider.swift:60およびios/TabViewImpl.swift:350にある2つのiOSの呼び出し箇所から始めてください。navigator-level defaultとper-screen overrideが、proposalで言及されているAndroidのlayoutに関する懸念を含め、両方の固定サイズの経路にどのように渡るかを確認してください。選択したAPIによって1つのtab iconがデフォルトの動作を壊さずに異なるサイズを使用できれば完了です。

索引モデルが issue の本文から書いたものです。