Skip 5 seconds back, drop picture in picture

[kvachikk]

What changed

• Back skips 5 seconds, forward stays 10. Going back is for the line of
  dialogue you missed; going forward is for the opening you have seen. A
  double-tap in each zone follows its own button: 5 seconds a tap on the left,
  10 on the right.
• Picture in picture removed. Gecko has no floating-window API on Android,
  so the button asked the system for the home screen and hoped — it worked on
  some builds and did nothing on others. The top row is one button shorter.
• Version 0.6.0, changelog and README updated.

A screenshot button was attempted and is not possible

Firefox for Android does not give a decoded video frame to any script:

• drawImage(video) into a 2D canvas leaves it fully transparent;
• uploading the video to a WebGL texture reads back as one flat colour,
  #330033, edge to edge;
• tabs.captureVisibleTab (tried with <all_urls>) renders the page with the
  video area black.

Measured from a page's own script, from a content script and from a background
script, on Firefox 153 on an AOSP Android 16 emulator and on a Pixel 9 Pro. The
button is therefore not shipped rather than shipped broken; the limitation is
written down in the README and the changelog.

Checks

• 60 unit tests pass
• eslint, prettier
• web-ext lint: 0 errors, 0 warnings
• privacy check passes; permissions unchanged (storage, activeTab)
• on the emulator: the player draws 5 on the left skip button and 10 on the
  right, and both seek by those amounts