Fix "Save As" not appending format extension for language-coded filenames

[ivandrofly]

Summary

• When extracting subtitles from MKV with a two-letter language code (e.g., .sv for Swedish), the "Save As" dialog suggested a filename without the subtitle format extension (e.g., .srt)
• Windows treated the language code as the file extension, resulting in files saved as name.sv instead of name.sv.srt
• Added subtitle format extension to the suggested filename in SaveSubtitleAs()
• Added DefaultExtension to FilePickerSaveOptions in PickSaveSubtitleFileAs(), matching the existing behavior in PickSaveSubtitleFile()

Test plan

• Open an MKV file containing multiple subtitle tracks with different languages (e.g., Swedish .sv and English .en)
• Select the Swedish subtitle track, press "Save As" — verify the suggested filename ends with .sv.srt (not just .sv)
• Select the English subtitle track, press "Save As" — verify the suggested filename ends with .en.srt
• Open a regular .srt file, press "Save As" — verify the suggested filename still ends with .srt (no double extension)

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