feat: add post-processing pass and video settings menu

Enable glow/bloom, color adjustments, and MSAA+FXAA across all three
arenas so emissive ship/station accents actually bleed light and edges
read cleanly. Expose the player-facing knobs (anti-aliasing mode, glow
intensity, brightness) through a new Settings screen off the main menu,
persisted via a VideoSettings autoload that scales each arena's own
tuned Environment values on load rather than overwriting them.

Game/scenes/arena_02.tscn

@@ -21,6 +21,22 @@ sky = SubResource("Sky_nebula")
 ambient_light_source = 2
 ambient_light_color = Color(0.65, 0.35, 0.75, 1)
 ambient_light_energy = 0.45
+glow_enabled = true
+glow_levels/1 = 0.0
+glow_levels/2 = 0.4
+glow_levels/3 = 0.8
+glow_levels/4 = 0.6
+glow_levels/5 = 0.2
+glow_intensity = 0.8
+glow_strength = 1.0
+glow_bloom = 0.03
+glow_blend_mode = 1
+glow_hdr_threshold = 1.1
+glow_hdr_scale = 2.0
+adjustment_enabled = true
+adjustment_brightness = 1.0
+adjustment_contrast = 1.05
+adjustment_saturation = 1.08
 
 [sub_resource type="StandardMaterial3D" id="StandardMaterial3D_dust"]
 transparency = 1