fix(css): native select popups follow the theme — color-scheme per scheme + themed option entries

The stats page's Library-tab switcher (and every other native
<select>) rendered its dropdown LIST in the OS light scheme in dark
themes: the closed control is themed by its own classes, but the
popup list is UA-rendered and ignores page CSS unless the document
declares color-scheme (Chrome path) or the options carry explicit
colors (Firefox path). Both are now in place:

- :root { color-scheme: dark } (the default palette — Tokyo Night —
  and all eleven themes except latte are dark),
- .theme-catppuccin-latte { color-scheme: light },
- select option { background-color: var(--bg-secondary); color:
  var(--text-primary) } as the belt for engines where the popup
  escapes the document cascade.

style.css regenerated via build:css:prod; verified served by the
rebuilt container.

web/static/input.css

@@ -10,6 +10,12 @@
    * theme-flash on load).
    */
   :root {
+    /* Native form controls (select dropdown lists, scrollbars, inputs)
+       follow the OS light scheme without this — the stats library
+       switcher's popup rendered white in every dark theme. Dark is the
+       document default (all themes except latte are dark). */
+    color-scheme: dark;
+
     --bg-primary: #1a1b26;
     --bg-secondary: #16161e;
     --text-primary: #c0caf5;
@@ -115,6 +121,8 @@
   }
 
   .theme-catppuccin-latte {
+    color-scheme: light;
+
     --bg-primary: #eff1f5;
     --bg-secondary: #ffffff;
     --text-primary: #4c4f69;
@@ -158,6 +166,13 @@
     --shadow-bar: 0 1px 0 rgba(76, 79, 105, 0.08), 0 4px 16px rgba(76, 79, 105, 0.10);
   }
 
+  /* Select popup entries follow the theme (the closed control is
+     themed by its own classes; the option LIST is what UA-styles light) */
+  select option {
+    background-color: var(--bg-secondary);
+    color: var(--text-primary);
+  }
+
   /* App-wide keyboard focus affordance (accessibility) */
   :focus-visible {
     outline: 2px solid var(--accent);