fix(QExpansionItem): switch-toggle-side colors are now correct

When using switch-toggle-side on QExpansionItem with an :icon prop, the
icon was turning grey and the expand arrow was turning black — the opposite
of the expected behavior (#18008).

Root cause: QItem.sass applies `color: $grey-7` to all `--side` sections
generically. When switchToggleSide is off the arrow's QItemSection uses
`side=true`; when switched on it uses `avatar=true`. Swapping those JS
props (as attempted in the previous commit) broke layout indentation and
min-width because `avatar`/`side` control geometry, not just color.

Fix: CSS-only approach using :has() selectors in QExpansionItem.sass:
- The avatar section wrapping the toggle icon is forced to $grey-7 (grey)
- The side section wrapping a regular icon is forced to inherit color (full)

This fixes colors without touching the JS layout props whatsoever, so
indentation, min-width, and padding remain correct in all configurations.

Closes #18008

Author: Jadu07

ui/src/components/expansion-item/QExpansionItem.sass

@@ -1,5 +1,14 @@
 .q-expansion-item
 
+  .q-item__section--avatar:has(> &__toggle-icon)
+    color: $grey-7
+
+  .q-item--dark .q-item__section--avatar:has(> &__toggle-icon)
+    color: rgba(255,255,255,.7)
+
+  .q-item__section--side:has(> .q-icon):not(:has(> &__toggle-icon))
+    color: inherit !important
+
   &__border
     opacity: 0