feat(desktop): add boxless text button variant; use for aux-model actions

New reusable `text` variant renders a button as inline label text (no
bg/border, muted -> foreground, underline-on-hover affordance). Emphasize the
actionable word by adding `font-semibold`/`underline` at the call site. Applied
to the auxiliary-model "Set to main" (plain), "Change" and "Reset all to main"
(bold + underlined) actions, replacing the boxed ghost/outline buttons.

apps/desktop/src/app/settings/model-settings.tsx

@@ -244,10 +244,11 @@ export function ModelSettings({ onMainModelChanged }: ModelSettingsProps) {
         <div className="mb-2.5 flex items-center justify-between">
           <SectionHeading icon={Cpu} title="Auxiliary models" />
           <Button
+            className="font-semibold underline"
             disabled={!mainModel || applying}
             onClick={() => void resetAuxiliaryModels()}
             size="sm"
-            variant="outline"
+            variant="text"
           >
             Reset all to main
           </Button>
@@ -270,15 +271,16 @@ export function ModelSettings({ onMainModelChanged }: ModelSettingsProps) {
                         disabled={!mainModel || applying}
                         onClick={() => void setAuxiliaryToMain(meta.key)}
                         size="sm"
-                        variant="ghost"
+                        variant="text"
                       >
                         Set to main
                       </Button>
                       <Button
+                        className="font-semibold underline"
                         disabled={!providers.length || applying}
                         onClick={() => beginAuxiliaryEdit(meta.key)}
                         size="sm"
-                        variant="outline"
+                        variant="text"
                       >
                         Change
                       </Button>