fix: theme toggle now properly handles auto mode

Remove data-theme attribute when mode="auto" instead of setting it to
"auto". Pico's OS-dark selector is :root:not([data-theme]), so any
value (even "auto") prevents dark-mode Pico styles. Add Pico form
element background vars and color-scheme: light to match active
theme.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

imptune/static/app.css

@@ -8,7 +8,9 @@
 
    Theme selectors mirror Pico's own so equal specificity + later source order
    wins:  light = :root:not([data-theme=dark]) ; dark = the two dark selectors.
-   `data-theme="auto"` is not `light`, so it follows the OS preference.
+   The "system" state renders with **no** data-theme attribute (base.html
+   removes it) — Pico's OS-dark block is `:root:not([data-theme])`, so a literal
+   data-theme="auto" would keep Pico light while this file went dark.
    ========================================================================== */
 
 /* --- Tokens: light (paper) ----------------------------------------------- */
@@ -73,9 +75,15 @@
   --pico-form-element-placeholder-color: var(--im-text-faint);
   --pico-form-element-active-border-color: var(--im-accent);
   --pico-form-element-focus-color: var(--im-accent);
+  /* Pico swaps the *background* on :focus/:active. Remap it too, or a focused
+     field falls back to Pico's own palette instead of ours. */
+  --pico-form-element-active-background-color: var(--im-surface);
+  --pico-form-element-selected-background-color: var(--im-surface-3);
   --pico-code-background-color: var(--im-surface-3);
   --pico-code-color: var(--im-text);
   --pico-modal-overlay-background-color: rgba(24, 30, 36, .42);
+  /* Native widgets: <select> popup, scrollbars, autofill. */
+  color-scheme: light;
 }
 
 /* --- Tokens: dark (ink) -------------------------------------------------- */
@@ -140,9 +148,12 @@
     --pico-form-element-placeholder-color: var(--im-text-faint);
     --pico-form-element-active-border-color: var(--im-accent);
     --pico-form-element-focus-color: var(--im-accent);
+    --pico-form-element-active-background-color: var(--im-surface-3);
+    --pico-form-element-selected-background-color: var(--im-surface-3);
     --pico-code-background-color: var(--im-surface-3);
     --pico-code-color: var(--im-text);
     --pico-modal-overlay-background-color: rgba(4, 8, 11, .62);
+    color-scheme: dark;
   }
 }
 
@@ -206,9 +217,12 @@
   --pico-form-element-placeholder-color: var(--im-text-faint);
   --pico-form-element-active-border-color: var(--im-accent);
   --pico-form-element-focus-color: var(--im-accent);
+  --pico-form-element-active-background-color: var(--im-surface-3);
+  --pico-form-element-selected-background-color: var(--im-surface-3);
   --pico-code-background-color: var(--im-surface-3);
   --pico-code-color: var(--im-text);
   --pico-modal-overlay-background-color: rgba(4, 8, 11, .62);
+  color-scheme: dark;
 }
 
 /* --- Typography + global rhythm ----------------------------------------- */