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/app/globals.css

@@ -590,6 +590,148 @@ html.dark body.site-bg:has([data-admin]) {
 	background-blend-mode: normal;
 }
 
+/* ── Animated site backdrop (Theme Manager effects) ────────────────
+   A fixed, decorative layer between the body wallpaper and the content.
+   Theme-driven: aurora + particles are pure CSS on transform/opacity, so
+   they stay cheap and pause for prefers-reduced-motion automatically. */
+.site-backdrop {
+	position: fixed;
+	inset: 0;
+	z-index: -1;
+	overflow: hidden;
+	pointer-events: none;
+	contain: layout paint style;
+}
+.site-backdrop > span {
+	position: absolute;
+	inset: 0;
+	pointer-events: none;
+}
+body:has([data-admin]) .site-backdrop {
+	display: none;
+}
+
+/* Aurora blobs (large, soft radial gradients — no filters). */
+.site-backdrop > .site-backdrop-aurora {
+	inset: -12% -10%;
+	overflow: hidden;
+}
+.site-backdrop-blob {
+	position: absolute;
+	border-radius: 999px;
+	opacity: 0.42;
+	will-change: transform;
+	animation: aurora-drift 20s ease-in-out infinite alternate;
+}
+.site-backdrop-blob-1 {
+	width: clamp(320px, 44vw, 620px);
+	height: clamp(320px, 44vw, 620px);
+	top: -14%;
+	left: -10%;
+	background: radial-gradient(
+		circle at 32% 32%,
+		color-mix(in srgb, var(--color-primary) 60%, transparent) 0%,
+		transparent 68%
+	);
+}
+.site-backdrop-blob-2 {
+	width: clamp(280px, 38vw, 540px);
+	height: clamp(280px, 38vw, 540px);
+	top: -10%;
+	right: -8%;
+	background: radial-gradient(
+		circle at 62% 40%,
+		color-mix(in srgb, var(--color-accent) 48%, transparent) 0%,
+		transparent 68%
+	);
+	animation-delay: -7s;
+	animation-duration: 24s;
+}
+.site-backdrop-blob-3 {
+	width: clamp(360px, 50vw, 700px);
+	height: clamp(360px, 50vw, 700px);
+	bottom: -22%;
+	left: 12%;
+	background: radial-gradient(
+		circle at 50% 50%,
+		color-mix(in srgb, var(--color-primary) 34%, transparent) 0%,
+		color-mix(in srgb, var(--color-accent) 18%, transparent) 42%,
+		transparent 72%
+	);
+	animation-delay: -13s;
+	animation-duration: 28s;
+}
+@keyframes aurora-drift {
+	0% {
+		transform: translate3d(0, 0, 0) scale(1) rotate(0deg);
+	}
+	100% {
+		transform: translate3d(6vw, 5vh, 0) scale(1.16) rotate(6deg);
+	}
+}
+html.dark .site-backdrop-blob {
+	opacity: 0.55;
+}
+
+/* Floating particles drifting upward. */
+.site-backdrop-particles {
+	inset: 0;
+}
+.site-backdrop-dot {
+	position: absolute;
+	bottom: -4vh;
+	width: calc(var(--dot-size, 3) * 1px + 1px);
+	height: calc(var(--dot-size, 3) * 1px + 1px);
+	border-radius: 999px;
+	background: color-mix(in srgb, var(--color-primary) 70%, transparent);
+	box-shadow: 0 0 8px color-mix(in srgb, var(--color-primary) 55%, transparent);
+	opacity: 0;
+	animation: particle-rise var(--dot-duration, 12s) linear infinite;
+	animation-delay: var(--dot-delay, 0s);
+}
+@keyframes particle-rise {
+	0% {
+		transform: translate3d(0, 0, 0);
+		opacity: 0;
+	}
+	10% {
+		opacity: var(--dot-alpha, 0.45);
+	}
+	88% {
+		opacity: var(--dot-alpha, 0.45);
+	}
+	100% {
+		transform: translate3d(var(--dot-drift, 2vw), -108vh, 0);
+		opacity: 0;
+	}
+}
+html.dark .site-backdrop-dot {
+	background: color-mix(in srgb, var(--color-primary) 82%, transparent);
+}
+
+/* Overlay tint above image/effects keeps custom backgrounds readable. */
+.site-backdrop-overlay {
+	inset: 0;
+	background: var(--background-overlay, transparent);
+}
+
+/* Show only the selected effect(s). */
+.site-backdrop[data-effect="none"] .site-backdrop-particles,
+.site-backdrop[data-effect="none"] .site-backdrop-aurora,
+.site-backdrop[data-effect="aurora"] .site-backdrop-particles,
+.site-backdrop[data-effect="particles"] .site-backdrop-aurora {
+	display: none;
+}
+@media (prefers-reduced-motion: reduce) {
+	.site-backdrop-blob {
+		animation: none;
+	}
+	.site-backdrop-dot {
+		animation: none;
+		opacity: 0;
+	}
+}
+
 .article-body {
 	line-height: 1.7;
 }