feat: global overview widget, optimized cache, CSS asset docs

- Add ShortUrlGlobalOverview StatsOverviewWidget with split cache strategy:
  link counts cached forever (invalidated on create/delete), click stats
  with short TTL (geo_ip.stats_cache_ttl config)
- Fix visited_at column in ShortUrlVisit queries (was created_at)
- Disable Livewire polling (pollingInterval = null)
- Add cache invalidation for global overview in ShortUrl::booted()
- Rewrite README asset section: user-facing install instructions clarified,
  post-autoload-dump tip added, developer recompile notes separated

resources/css/plugin.css

@@ -0,0 +1,17 @@
+@import "tailwindcss";
+
+@source "../../src/**/*.php";
+@source "../../resources/views/**/*.blade.php";
+
+.short-url-card {
+    position: relative !important;
+}
+
+.short-url-card .fi-ta-actions {
+    position: absolute !important;
+    top: 1.25rem !important;
+    right: 1.25rem !important;
+    z-index: 10 !important;
+    margin: 0 !important;
+    padding: 0 !important;
+}