Precise frame player and a new media core.

• Frames are confirmed through requestVideoFrameCallback: VFR videos step by time, "every frame" is offered only for CFR.
• Accessible frame markers with previews on the seek bar (mouse and keyboard).
• Playback speed is remembered per video.
• HEVC and AV1 save to fMP4 without re-encoding; AV1 in MPEG-TS stays .ts instead of being passed off as MP4.
• OPFS mode is deliberately off by default — the existing path stays primary and as fallback.
• Ukrainian and Belarusian locales; accessibility: focus trap, aria-live progress, keyboard tab navigation.
• Versioned message protocol v2 and settings migrations.

Major update from 1.3.2 to 1.7.0.

— Audio-only M4A with tags: title, author, album/playlist, episode number and cover art.
— Resume after a failure or browser restart; unfinished downloads are shown in the popup.
— Playlists: pick episodes with checkboxes, see titles/durations, a selected counter and size estimate.
— Subtitles next to the video with one checkbox, filename template and default format.
— History of the last 30 saved files in the popup.
— Video chapters and panel improvements. Still fully local: no servers, analytics or trackers.

Store preparation.