feat(theme): animate public background with aurora and particles, polish landing pages

Add background_effect (aurora/particles), background_overlay tint and
opacity to the theme manager, rendered site-wide by ThemeVars on every
public page. Polish the home and register pages (hero mascot, live stat
pulse, date pills, photo strip, CTA band, theme-aware register intro,
i18n for home/register section).

src/lib/theme-bg.ts

@@ -7,9 +7,37 @@ export const BACKGROUND_KEYS = [
 	"background_position",
 	"background_size",
 	"background_repeat",
+	"background_effect",
+	"background_overlay",
+	"background_overlay_opacity",
 ] as const;
 export type BackgroundKey = (typeof BACKGROUND_KEYS)[number];
 
+/**
+ * Animated background effects rendered behind the public site. Purely
+ * decorative and CSS-driven (transform/opacity only) so they stay cheap on
+ * every device and respect `prefers-reduced-motion`.
+ */
+export const BACKGROUND_EFFECTS = [
+	"none",
+	"aurora",
+	"particles",
+	"aurora-particles",
+] as const;
+export type BackgroundEffect = (typeof BACKGROUND_EFFECTS)[number];
+
+export const BACKGROUND_EFFECT_DEFAULT: BackgroundEffect = "none";
+
+export const OVERLAY_PRESETS = [
+	{ label: "None", value: "" },
+	{ label: "Black", value: "#000000" },
+	{ label: "White", value: "#ffffff" },
+	{ label: "Navy", value: "#0b1220" },
+	{ label: "Slate", value: "#1e293b" },
+] as const;
+
+export const OVERLAY_OPACITY_MAX = 90;
+
 export const BACKGROUND_POSITIONS = [
 	"center center",
 	"top center",
@@ -36,6 +64,9 @@ export const BACKGROUND_DEFAULTS: Record<BackgroundKey, string> = {
 	background_position: "center center",
 	background_size: "cover",
 	background_repeat: "no-repeat",
+	background_effect: BACKGROUND_EFFECT_DEFAULT,
+	background_overlay: "",
+	background_overlay_opacity: "0",
 };
 
 /**