Match the header's search and notification triggers - #20106

Description

The header's search and notification triggers now match: both are small plain icon buttons with full-size icons, grouped apart from the user menu. Icon size comes from a new --c-icon-size custom property, which also reaches icons nested inside slotted content.

 craft-icon
-  font-size: 0.8em
+  font-size: var(--c-icon-size, 0.8em)

 craft-button  (ellipsis icons)
-  font-size: 1em
+  --c-icon-size: 1em

 CommandPaletteTrigger  (search, was 0.8em)
+  --c-icon-size: 1em

 cp-notification-center  (bell, was 1.1em on the icon)
-  variant="none", .notification-trigger__icon { font-size: 1.1em }
+  variant="plain" icon, --c-icon-size: 1em

Evidence

• Before: search 10.08px, bell 15.4px
  
  After: both 12.6px (1em of the small button)
  

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