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>

tests/e2e/test_theme_toggle.py

@@ -7,9 +7,11 @@ def test_theme_cycles_on_click(page, live_server: str) -> None:
     """Clicking theme button cycles data-theme attribute: auto -> light -> dark -> auto."""
     page.goto(f"{live_server}/", wait_until="domcontentloaded")
 
-    # Initial state: auto (default from base.html)
+    # Initial state: auto — the attribute must be ABSENT, not "auto". Pico's
+    # OS-dark block is keyed on `:root:not([data-theme])`, so any value pins
+    # Pico to its light theme while app.css follows the OS into dark.
     initial_theme = page.evaluate("document.documentElement.getAttribute('data-theme')")
-    assert initial_theme == "auto"
+    assert initial_theme is None
 
     # Click once -> light
     page.click("button[aria-label='auto']")
@@ -27,6 +29,13 @@ def test_theme_cycles_on_click(page, live_server: str) -> None:
     )
     assert page.evaluate("document.documentElement.getAttribute('data-theme')") == "dark"
 
+    # Click again -> auto, which removes the attribute again
+    page.click("button[aria-label='dark']")
+    page.wait_for_function(
+        "!document.documentElement.hasAttribute('data-theme')",
+        timeout=2000,
+    )
+
 
 def test_theme_persists_across_reload(page, live_server: str) -> None:
     """After clicking theme toggle, the chosen theme is restored on reload."""