feat: unhide action, date-range filter, more sort options

- Hidden view shows an Unhide action (eye icon) instead of Hide
- Upload-date filter: From/To date range (inclusive); feed shows only videos
  published in that window (backend published_after / published_before)
- New sort options: Name (A-Z) and Channel subscribers, alongside date/views/
  duration/shuffle
- Native controls follow the theme via color-scheme (dark date picker)

frontend/src/index.css

@@ -6,6 +6,14 @@
   --font-scale: 1.06;
 }
 
+/* Make native controls (date picker, scrollbars) follow the theme. */
+html[data-theme="dark"] {
+  color-scheme: dark;
+}
+html[data-theme="light"] {
+  color-scheme: light;
+}
+
 html {
   font-size: calc(16px * var(--font-scale));
   background: var(--bg);