Automatically detect and display external subtitles

## Problem
Available external subtitles are not automatically detected and displayed by the app when playing a video file.

## Proposal
When start playing a video, check if there are any external subtitle file(s) available in the same folder and with the same file name (but with different extension) and automatically display them when playback starts.

## Prior Art
VLC will automatically open and play external subtitle files that are in the same folder as the played video and which filename matches the video file.

No files, tests, or entry points are named in the issue. Start by locating the video playback-start handling and the code that discovers or loads subtitle files, then trace how the current video path is exposed. Done means matching external subtitle files in the video's folder are detected and displayed automatically when playback begins.

Written by the indexing model from the issue text.