Add volume slider

[adotdong29]

Hi! I noticed #60 was tagged good first issue and figured I'd take a stab. Long-time setwithforks player, first-time contributor. I'm happy to iterate on this if anything needs adjusting, or recieve any feedback!

Closes #60.

Behavior

• Replaces the binary "Mute / Unmute sound" menu item with a 0-100 slider plus a speaker icon button.
• The slider sets the actual playback volume of the success/fail SFX through use-sound's volume option, so 30% really plays at 30%.
• The speaker icon and the existing Ctrl+Shift+S shortcut both toggle mute. They remember the last non-zero level so toggling back doesn't snap to 100%.
• Old "on" / "off" values stored in localStorage are migrated transparently to 100 / 0 the first time parseVolume reads them. Existing users see no change unless they move the slider.

Notes

• Step is set to 5 on the slider — felt about right for a quick mouse-drag, but easy to drop to 1 if you'd prefer finer control.
• Left the Help page shortcut text as "Mute or unmute sound" since the binding still does that. Happy to rename it if you'd rather call it "Toggle mute" or similar.