Add numeric jump shortcuts for track seeking

[paulpv]

Problem

• Quickly jumping to a rough position in a long track currently requires dragging the seek bar, which is slow and imprecise compared to keyboard shortcuts.
• Desktop users expect the YouTube-style number keys (0-9) to snap playback to 10% increments of the track, but the player does not support this today.

Proposal

• Bind the top-row number keys 0 through 9 so pressing the key seeks to (digit * 10)% of the current track duration (0 = start, 9 = 90%).
• Ensure the shortcuts only fire when the focus is on the player (not when typing in dialogs or other inputs).
• Provide a tooltip or README note so the new binding is discoverable.

Acceptance Criteria

• Pressing a number key while the player has focus jumps playback to the expected percentage of the track length.
• Shortcuts work consistently across Chromium-based browsers on macOS, Windows, and Linux.
• Existing keyboard navigation (e.g., arrow keys, space/enter) keeps working without regressions.

Notes

• Matching the YouTube behavior will make long-form listening easier and keeps keyboard expectations consistent across media players.