feat: add configurable double-tap seek with ytp-seek visual feedback

[heidevary]

Summary

Implements configurable double-tap seek distances for touch devices, addressing issue #4132.

This PR follows the maintainer's guidance from the issue discussion:

• Touch inputs added to shortcuts.js - the touchend listener is integrated into the existing shortcutsListeners system
• ytp-seek-* visual classes modified through the DOM - toggles ytp-seek-forward-bump / ytp-seek-backward-bump on the player element, and triggers YouTube's ytp-doubletap-ui overlay
• Uses player.seekTo() - as the maintainer specified, not seekBy()
• event.preventDefault() + event.stopPropagation() - prevents YouTube's default double-tap behavior when custom mode is active

Features

Three seek modes (as requested in the issue):

1. Default - YouTube's native behavior (no interception)
2. Fixed distance - user-configurable seek distance (1-60 seconds), same amount every tap
3. Progressive distance - separate distances for double-tap, triple-tap, and quadruple-tap

Changes

File	Change
shortcuts.js	Add touchend listener to shortcutsListeners, doubleTapSeek() function, modify shortcutsInit()
menu/skeleton-parts/player.js	Add Double-tap seek section with mode selector and distance sliders
_locales/en/messages.json	Add i18n labels for new settings
tests/unit/double-tap-seek.test.js	Jest tests for seek logic, boundary checks, progressive mode, visual feedback

/claim #4132

[heidevary]

Self-Check Fix

After reviewing my own PR, I found and fixed one issue:

shortcuts.js - Fixed preventDefault() breaking touch controls

Problem: The touchend handler called event.preventDefault() and event.stopPropagation() on all touchend events in the player area. This would break touch interactions with player controls (play/pause button, fullscreen, settings gear, etc.).

Fix: Added a check before the preventDefault logic:

const target = event.target;
if (target && target.closest && target.closest('.ytp-chrome-bottom, .ytp-chrome-top, .ytp-settings-menu, .ytp-popup, .ytp-button, button, a')) return;

This skips interception when the touch target is inside a player control element, allowing normal touch interaction while still enabling double-tap seek on the video surface.