Add a Nord theme and give the player bar room around it (#43)

Nord (themes/nord_theme.tres) uses the app icon's blues, darkened, with Nord's
Snow Storm text, Frost and Aurora accents and the playing song in Aurora
yellow. Its shapes are round: pill buttons and tabs, 14-16 px corners, soft
shadows on popups, cards and the player bar, rounded scroll bars, sliders and
text fields, and seven Aurora/Frost colours for the score graph. Classic stays
the default.

The player bar now has 16 px to the song buttons, the player list, Quiz
settings and the window's bottom edge, and the right edges line up at 1856.

The volume icon is a flat button with a PlayerIcon style, so each theme colours
it; it was nearly invisible on Original web's light grey bar.

Closes #43

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

player/MusicPlayer.tscn

@@ -109,14 +109,16 @@ theme_type_variation = &"PlayerTime"
 text = "00:00 / 00:00"
 horizontal_alignment = 1
 
-[node name="VolumeIcon" type="TextureRect" parent="PlayerBar/MusicPlayerContainer"]
+[node name="VolumeIcon" type="Button" parent="PlayerBar/MusicPlayerContainer"]
 custom_minimum_size = Vector2(22, 22)
 layout_mode = 2
 size_flags_vertical = 4
 mouse_filter = 2
-texture = ExtResource("6_volume")
-expand_mode = 1
-stretch_mode = 5
+focus_mode = 0
+theme_type_variation = &"PlayerIcon"
+icon = ExtResource("6_volume")
+icon_alignment = 1
+expand_icon = true
 
 [node name="VolumeSlider" parent="PlayerBar/MusicPlayerContainer" instance=ExtResource("5_iifuj")]
 unique_name_in_owner = true