Add silent-gap removal during playback

[maboa]

Summary

• Adds a "Remove Silent Gaps" feature: during playback, automatically skip silences between words that exceed a threshold, preserving a small buffer on each side
  so word edges aren't clipped.
• New toolbar button (Lucide rabbit icon) opens a modal with a single on/off toggle. Threshold (ms) and buffer (ms) are tucked behind an "Advanced settings"
  disclosure so the default UI stays simple.
• Settings apply immediately on change — no Apply/Cancel needed.
• When enabled, the modal shows "Playback shortened by X.Y seconds" computed from the current transcript and settings, updating live as you tweak them.
• A small primary-colored dot in the corner of the toolbar button indicates when gap removal is active.
• Plays well with existing strikethrough/redaction logic — gap skipping is layered on top of the same playable-section computation.

Test plan

• Load a transcript with audio that contains noticeable silences between words.
• Open the rabbit modal, enable "Skip gaps during playback", and confirm playback skips long gaps.
• Tweak threshold and buffer in Advanced settings and verify the "Playback shortened by X.Y seconds" figure updates live.
• Verify the purple dot on the toolbar button shows when enabled and hides when disabled.
• Toggle some words to struck-through (redacted) and confirm both struck spans and silent gaps are skipped.
• Reload a transcript while the toggle is on and verify gap skipping still works on the freshly loaded content.
• Close and reopen the modal; verify the toggle state, advanced inputs, and savings message reflect current settings.

Addresses #272