bug: "SignerContext" missing export in @miden-sdk/miden-wallet-adapter-react v0.15.5 [1.1.1]

### Describe the Bug
When attempting to build a Vite + TypeScript project that includes `@miden-sdk/miden-wallet-adapter-react` as a dependency, the bundler (Vite/Rollup/Rolldown) fails during the build process due to a missing export error from a peer dependency [1.1.1, 1.2.1]:

`[MISSING_EXPORT] "SignerContext" is not exported by "__vite-optional-peer-dep:@miden-sdk/react:@miden-sdk/miden-wallet-adapter-react"`

This occurs because `MidenFiSignerProvider.js` at line 3 attempts to import `SignerContext` from `@miden-sdk/react`, but that export is either missing or has a version mismatch in the current npm release [1.1.1]:
`import { SignerContext } from '@miden-sdk/react';`

### Steps to Reproduce
1. Initialize a new Vite + React + TypeScript project.
2. Install `@miden-sdk/miden-wallet-adapter` and `@miden-sdk/miden-wallet-adapter-react` (v0.15.5) [1.1.1, 1.2.8].
3. Run `npm run build` or `vite build`.
4. The build fails with the `[MISSING_EXPORT]` error originating from `MidenFiSignerProvider.js` [1.1.1].

### Environment & Package Versions
- **Vite version:** `v8.2.1` or modern Vite bundlers.
- **@miden-sdk/miden-wallet-adapter-react:** `v0.15.5` [1.1.1]
- **@miden-sdk/miden-wallet-adapter:** `v0.15.5` [1.2.8]

### Reproduction Repository
This build failure can be reproduced inside my public SKS Portal workspace:
🔗 https://github.com/anaraydinli55/miden-testnet-project

检查 MidenFiSignerProvider.js 文件的第 3 行，查看 import 语句。查看 @miden-sdk/react 包的导出，确认 SignerContext 是否可用。比较包之间的版本。通过更新导入或依赖版本来测试修复，然后从复现仓库运行构建命令进行验证。

由索引模型根据 Issue 内容生成。