Add @ command handling to useGeminiStream (#217)

* First integration of at commands into useGeminiStream.ts

* feat: Integrate @ command for file/directory reading

   - Adds support for `@<path>` commands in the CLI UI to read file or directory contents using the `read_many_files` tool.
   - Refactors `useGeminiStream` hook to handle slash, passthrough, and @ commands before sending queries to the Gemini API.
   - Improves history item ID generation to prevent React duplicate key warnings.

* fix: Handle additional text after @ command path

   - Modifies the `@` command processor to parse text following the file/directory path (e.g., `@README.md explain this`).
   - Includes both the fetched file content and the subsequent text in the query sent to the Gemini API.
   - Resolves the TODO item in `atCommandProcessor.ts`.

* feat: Allow @ command anywhere in query and fix build

   - Update `atCommandProcessor` to correctly parse `@<path>` commands regardless of their position in the input string using regex. This enables queries like "Explain @README.md to me".
   - Fix build error in `useGeminiStream` by importing the missing `findSafeSplitPoint` function.

* rename isPotentiallyAtCommand to isAtCommand

* respond to review comments.

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