feat: visual polish & UX — fade-in, theme toggle, scroll-to-top, footer, cursor, badge

- Add FadeIn component (IntersectionObserver scroll-reveal + load animation)
- Add dark/light theme toggle (useSyncExternalStore + localStorage)
- Add scroll-to-top button (appears after 300px scroll)
- Add configurable footer with social links (gallery.yaml)
- Add crosshair cursor on photo grid items
- Add 'X photos' count badge on subpage cover images
- Add light theme color palette in tokens.css

lib/env.ts

@@ -0,0 +1,45 @@
+/**
+ * Zod-validated environment variables.
+ * Parsed once at startup — all env access should go through this module.
+ */
+
+import { z } from 'zod';
+
+const envSchema = z.object({
+  /** Base URL of your Immich instance, e.g. https://immich.example.com */
+  IMMICH_API_URL: z
+    .string()
+    .url('IMMICH_API_URL must be a valid URL')
+    .transform((url) => url.replace(/\/+$/, '')),
+
+  /** Immich API key (from Admin → API Keys) */
+  IMMICH_API_KEY: z.string().min(1, 'IMMICH_API_KEY is required'),
+
+  /** Public site title shown in the header */
+  SITE_TITLE: z.string().default('Gallery'),
+
+  /** Subtitle displayed below the title */
+  SITE_SUBTITLE: z.string().default(''),
+
+  /** Cache TTL in seconds */
+  CACHE_TTL: z.coerce.number().int().min(0).default(300),
+
+  /** Rate limit: max requests per minute per IP */
+  RATE_LIMIT_RPM: z.coerce.number().int().min(1).default(120),
+});
+
+export type Env = z.infer<typeof envSchema>;
+
+function parseEnv(): Env {
+  const result = envSchema.safeParse(process.env);
+  if (!result.success) {
+    const formatted = result.error.issues
+      .map((issue) => `  ✗ ${issue.path.join('.')}: ${issue.message}`)
+      .join('\n');
+    throw new Error(`\n❌ Environment validation failed:\n${formatted}\n`);
+  }
+  return result.data;
+}
+
+/** Validated, typed environment variables. */
+export const env = parseEnv();