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

.env.local.example

@@ -1,17 +1,27 @@
-# Immich Server Connection
+# ── Immich Server Connection (required) ──────────────────────────
+# Validated by Zod at startup — missing or invalid values produce clear errors.
+
+# Base URL of your Immich instance (must be a valid URL)
 IMMICH_API_URL=http://your-immich-server:2283
+
+# Immich API key (from Admin → API Keys)
 IMMICH_API_KEY=your-api-key-here
 
-# Optional: Site title and subtitle (default: "Gallery")
+# ── Optional Settings ────────────────────────────────────────────
+# All have sensible defaults and are type-checked by Zod.
+
+# Site title shown in the header (string, default: "Gallery")
 SITE_TITLE=My Photography
+
+# Subtitle displayed below the title (string, default: "")
 SITE_SUBTITLE=A visual journal
 
-# Optional: Cache TTL in seconds (default: 300 = 5 minutes)
+# Cache TTL in seconds (integer >= 0, default: 300 = 5 minutes)
 CACHE_TTL=300
 
-# Optional: Rate limit for image proxy (requests per minute per IP, default: 120)
+# Rate limit for image proxy (integer >= 1, requests per minute per IP, default: 120)
 # RATE_LIMIT_RPM=120
 
-# ── Album configuration ──
+# ── Album configuration ──────────────────────────────────────────
 # Albums, subpages, hero image, and passwords are defined in content/gallery.yaml
 # See content/gallery.yaml.example for the full reference.