Fix small English typos in dialogs

[Bolajiadesina]

This PR fixes two small user-facing English text issues:

1. In the prompt about opening a new file when another file is already open, remove the duplicated word “already” in the message text:

   • Key: Ask about what to do when opening a new file when another file is already already open?
   • Value: Ask about what to do when opening a new file when another file is already open

2. In the subtitles import helper text, correct the typo “subtitiles” → “subtitles” while keeping the i18n key unchanged:

   • Key: Convert subtitiles into segments
   • Value: Convert subtitles into segments

No behavior changes, only minor text fixes in locales/en/translation.json.

[Bolajiadesina]

If you have time, please review this PR for me.