Strip filler words from transcriptions via configurable regex

[lucyfarnik]

Summary

• Adds post-processing step to remove filler sounds (um, uh, er, ah, hmm, mm) from transcription output
• Feature is off by default, toggled via "Remove Filler Words" in Transcription settings
• Regex pattern is user-editable so users can customize which words get filtered
• Applied in both the main window and the indicator window transcription paths

Test plan

• Verify transcriptions work normally with the feature disabled (default)
• Enable the toggle and verify filler words are stripped from output
• Edit the regex pattern and verify custom patterns work
• Verify whitespace is cleaned up properly after filler removal