fix(settings): match the provider combobox triggers to the select chrome

The provider Type combobox rendered as a Button outline — opaque bg-background,
a ChevronsUpDown icon and font-medium text — while every select on the knowledge
form is a transparent-bg SelectTrigger with a single ChevronDown. Side by side
the Type field read as a different control.

The combobox trigger now carries bg-transparent, px-3, font-normal and a
ChevronDown size-4, matching SelectTrigger; the model combobox's addon chevron
follows suit. Measured against the canon at 1440: transparent bg, chevron-down
16px opacity .5, h-9, px-3, weight 400 — identical.

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

frontend/src/pages/settings/settings-provider.tsx

@@ -3,7 +3,7 @@ import {
     AlertCircle,
     Check,
     CheckCircle,
-    ChevronsUpDown,
+    ChevronDown,
     Clock,
     Ellipsis,
     Lightbulb,
@@ -213,12 +213,15 @@ function FormComboboxItem<T extends FieldValues = FieldValues>({
                 >
                     <PopoverTrigger asChild>
                         <Button
-                            className={cn('w-full justify-between', !displayValue && 'text-muted-foreground')}
+                            className={cn(
+                                'h-9 w-full justify-between bg-transparent px-3 font-normal hover:bg-transparent',
+                                !displayValue && 'text-muted-foreground',
+                            )}
                             disabled={disabled}
                             variant="outline"
                         >
                             {displayValue || placeholder}
-                            <ChevronsUpDown className="opacity-50" />
+                            <ChevronDown className="size-4 opacity-50" />
                         </Button>
                     </PopoverTrigger>
                     <PopoverContent
@@ -464,7 +467,7 @@ function FormModelComboboxItem<T extends FieldValues = FieldValues>({
                                     disabled={disabled}
                                     size="icon-sm"
                                 >
-                                    <ChevronsUpDown className="opacity-50" />
+                                    <ChevronDown className="size-4 opacity-50" />
                                 </InputGroupButton>
                             </PopoverTrigger>
                         </InputGroupAddon>