File read and overwrite via subtitle URI

Path traversal in Streambert before 2.5.0 allows local users to copy readable files and overwrite writable files via subtitle downloads. In src/ipc/downloads.js, the downloadSubtitleFile utility reachable through the run-download IPC channel accepts a renderer-supplied file: URL, decodes its pathname, and passes it directly to fs.copyFileSync while also trusting the renderer-controlled destination path. Exploitation requires control of a renderer process, after which any file readable by the Streambert process can be copied into an attacker-chosen writable location and existing writable files can be replaced.