Renamed so npm run package:extension succeeds

[gitxpresso]

This pull request fixes this error:

> package:extension
> cd extension && npm run package


> package
> node bin/build.js --package

▲ [WARNING] Use "src/background/ReceiverSelector.ts" instead of "src/background/receiverSelector.ts" to avoid issues with case-sensitive file systems [different-path-case]

    src/background/castManager.ts:27:7:
      27 │ } from "./receiverSelector";
         ╵        ~~~~~~~~~~~~~~~~~~~~

✘ [ERROR] Cannot read file: /home/gitxpresso/Downloads/fx_cast/extension/src/background/receiverSelector.ts

    src/background/castManager.ts:27:7:
      27 │ } from "./receiverSelector";
         ╵        ~~~~~~~~~~~~~~~~~~~~

1 warning and 1 error
Build error!
/home/gitxpresso/Downloads/fx_cast/extension/node_modules/esbuild/lib/main.js:1467
  let error = new Error(text);
              ^

Error: Build failed with 1 error:
src/background/castManager.ts:27:7: ERROR: Cannot read file: /home/gitxpresso/Downloads/fx_cast/extension/src/background/receiverSelector.ts
    at failureErrorWithLog (/home/gitxpresso/Downloads/fx_cast/extension/node_modules/esbuild/lib/main.js:1467:15)
    at /home/gitxpresso/Downloads/fx_cast/extension/node_modules/esbuild/lib/main.js:926:25
    at /home/gitxpresso/Downloads/fx_cast/extension/node_modules/esbuild/lib/main.js:1345:9
    at process.processTicksAndRejections (node:internal/process/task_queues:103:5) {
  errors: [Getter/Setter],
  warnings: [Getter/Setter]
}

Node.js v22.22.2

whenever npm run package:extension is ran