Native Studio filler-word removal using local word-timed captions #2960

Description

Add native filler-word removal to the GPUI Studio editor so a recording can be cleaned up without exporting it to another editor or running an external script.

A local prototype used Cap's word-timed captions to remove whole “um” and “uh” spans by changing the editable clip timeline. It preserved the original media, camera and cursor tracks; the resulting project reopened and exported successfully, and the cuts were accepted after listening.

Proposed behavior

• Provide a Remove Filler Words action alongside the native Captions controls.
• Generate local word-timed captions automatically when they are missing, using the selected downloaded model.
• Start with explicit English support and whole um, uh, uhm, er and erm hesitation tokens, including drawn-out forms; preserve lexical substrings, acronyms and context-dependent words such as “like”.
• Apply editable timeline cuts with the existing ripple-edit and caption-projection helpers. Preserve source media and the source transcript.
• Group cleanup into one undo step. Repeating cleanup should not cut the same source span again.
• Keep the resulting clips fully editable: recover a boundary using trim handles, or split and delete a missed hesitation with the normal Studio tools.
• Preserve natural pauses around cuts. Where recognizer word ends extend to the next onset, use the audio to retain a detected pause and audible release; retain existing timings when speech cannot be separated from noise.
• Report skipped words near transitions/held frames and reject invalid or overlapping timestamps without partially changing the project.
• Keep caption visibility as selected by the user; generating a transcript for cleanup should not implicitly turn on visible subtitles.

This is transcript-based cleanup. Playback review is still needed because recognition and word boundaries can be imperfect. An optional cleanup preference for new recordings can be considered after the native action and timing behavior are verified.

Additional Context

• Target: the shipping GPUI desktop app (apps/desktop-gpui), Cap 0.6.1.
• Prior art: feat(desktop): transcript editing - soft delete with ability to restore, deleted words buffers, pause deletion #1880 explored transcript editing and word-end timing, but was closed unmerged following the move from Tauri to GPUI. The native feature should reuse current GPUI editing/transcription paths rather than port that retired UI.
• General execution of saved automation rules after desktop Studio recordings is tracked separately in GPUI: execute configured automation rules after Studio recordings #2959.
• Searched existing issues for filler words, hesitation and disfluency; no dedicated native GPUI feature issue was found.