SponsorBlock integration — auto-skip sponsored segments

Feature Request: SponsorBlock Integration

Description

Integrate SponsorBlock API to automatically skip sponsored segments, intros, outros, and other non-content sections in YouTube videos.

Why

This is the #1 missing feature in ImprovedTube. Competitors already have it:

• YouTube Alchemy (GreasyFork, 1.8K installs) — persistent progress bar with SB segments
• EasyTube (GreasyFork, 57K+ installs) — SponsorBlock integration baked in
• vBlockTube (GitHub) — full SponsorBlock support
• Focused YouTube (GreasyFork) — skip sponsored content

ImprovedTube has a planned issue (#1452) for this but no implementation yet.

Preferred behavior

• Fetch SponsorBlock segments via their public API
• Show segments on the progress bar (colored markers)
• Auto-skip or show skip button for each category
• Toggle per category (sponsor, intro, outro, self-promo, etc.)
• Local caching of segments for performance

Reference

• SponsorBlock API: https://sponsor.ajay.app/

The issue names no implementation files, entry points, or tests. Start by reading the existing YouTube player integration and the SponsorBlock API reference at https://sponsor.ajay.app/, then define the segment categories, progress-bar markers, skip controls, preferences, and caching behavior; done means these requested behaviors work together.