Feat: Implement Smart Speed feature using YouTube heatmap (#1463)

[AryaXDG]

This Pull Request implements the "Smart Speed" feature requested in Issue #1463. This functionality leverages YouTube's native "Most Replayed" heatmap data (engagement histogram) to dynamically adjust playback speed in real-time, optimizing content consumption efficiency.

The feature analyzes normalized engagement metrics to automatically:

1. Skip Intros: Seeks past low-engagement segments at the start of the video (0-15% duration) if the interest score is below the defined threshold.
2. Dynamic Playback Speed: Scales playback speed inversely to user engagement during low-retention sections.
3. Restore Normal Speed: Automatically reverts to 1.0x speed during high-engagement segments.

Key Changes:

• Logic Engine (player.js):
• Implemented ImprovedTube.heatmap module to encapsulate state and logic.
• Resilient Data Extraction: Developed a multi-tiered data retrieval strategy to handle YouTube's A/B testing and data lazy-loading:

1. Tier 1: Direct DOM access via getPlayerResponse().
2. Tier 2: Inspection of global window objects (ytInitialData, ytInitialPlayerResponse).
3. Tier 3: Background fetch of the document source to parse raw JSON when runtime data is unavailable or obfuscated (common in music videos or specific user sessions).

• Recursive Parsing: Implemented a recursive search algorithm to locate MARKER_TYPE_HEATMAP objects within arbitrary JSON structures, making the feature robust against minor DOM layout changes.

• Dynamic Speed Mapping: Implemented a heuristic gradient for speed adjustment based on intensity scores:
  Score ≤ 0.05: 2.00x
  Score ≤ 0.12: 1.75x
  Score ≤ 0.18: 1.50x
  Score ≤ 0.25: 1.25x
  Score > 0.25: 1.00x

• User Interface (menu/skeleton-parts/player.js):
  Added a toggle switch in the "Player" section labeled "Smart Speed (Skip Boring Parts)".

• Shortcuts (shortcuts.js):
  Added logic for a keyboard shortcut to toggle the feature state during playback.

• Localization (messages.json):
  Added the English locale string for the new menu item.

Technical Implementation Details:

• Performance: The logic loop utilizes a 500ms throttle to ensure negligible CPU impact during playback monitoring.
• Safety: Extensive try/catch blocks and type checking prevent player instability if upstream API data formats change.
• SPA Support: Includes event listeners for yt-navigate-finish to ensure correct re-initialization during YouTube's Single Page Application navigation events.

How to Test:

1. Enable "Smart Speed (Skip Boring Parts)" in the ImprovedTube Player menu.
2. Navigate to a video with established heatmap data (e.g., a popular tutorial or educational video).
3. Monitor the playback speed via the YouTube player settings. Verify that speed increases during low-engagement sections and returns to 1.0x during peaks.
4. Verify that intros with low engagement (first 15%) are automatically skipped.
5. Test on a video known to obscure data (e.g., music videos) to verify the background fetch fallback mechanism functions correctly.

Related Issue:
Closes #1463

[ImprovedTube]

Awesome!!!

( added the shortcut GUI element. )

isEnabled

• Sorry our global functions & documentation didn't spare you from these lines of work. ( Functions and settings can be separate like if (this.storage.player_always_repeat === true) { ImprovedTube.playerRepeat(); } )

[AryaXDG]

No worries at all! I expected a bit of legacy structure given the project's history, but it was honestly a fun challenge to figure out how everything fits together.