Unify disabled appearance across all form controls

Disabled controls looked inconsistent: the SearchSelect faded (opacity-50)
while native inputs used a solid strong surface. Standardize on the faded look
(opacity-50) the user preferred, via shared constants so every form element
matches:

- DISABLED_CONTROL_CLASS (disabled:opacity-50 disabled:cursor-not-allowed) on
  the control — native inputs/select/textarea via PrimitiveWidgetsMixin, plus
  the Checkbox component (previously had no disabled style).
- DISABLED_WITHIN_CLASS (has-[:disabled]: wrapper variant) for composite
  controls like SearchSelect whose disabled state lives on an inner element.

e2e asserts a disabled SearchSelect and the Name input fade identically
(opacity 0.5) and return to 1 when enabled. CLAUDE.md documents the shared
disabled constants.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

common/components/primitives.py

@@ -46,6 +46,13 @@ _SIZE_CLASSES = {
     "xl": "px-6 py-3.5 text-base",
 }
 
+# Shared disabled appearance for every form control, so all form elements look
+# the same when disabled. Put on the control itself (DISABLED_CONTROL_CLASS) or,
+# for composite controls whose disabled state lives on an inner element (e.g.
+# SearchSelect), on the wrapper via :has() (DISABLED_WITHIN_CLASS).
+DISABLED_CONTROL_CLASS = "disabled:opacity-50 disabled:cursor-not-allowed"
+DISABLED_WITHIN_CLASS = "has-[:disabled]:opacity-50 has-[:disabled]:cursor-not-allowed"
+
 
 # ── Generic leaf elements ────────────────────────────────────────────────────
 # A whitelist of plain tags, each turned into a builder over `Element`. The
@@ -404,7 +411,8 @@ def Checkbox(
         ("value", value),
         (
             "class",
-            "rounded border-default-medium bg-neutral-secondary-medium text-brand focus:ring-brand",
+            "rounded border-default-medium bg-neutral-secondary-medium "
+            f"text-brand focus:ring-brand {DISABLED_CONTROL_CLASS}",
         ),
     ] + attributes
     if checked: