ui : add a tertiary variant and give secondary its own look back

The muted fill belonged to the filter box, but the chat form's add button
and the app's cancel buttons share the secondary variant. That look moves
to tertiary, which the manage dialog's calls to action use, and secondary
returns to the lighter surface it had before.

Assisted-by: pi:llama.cpp/DeepSeek-V4.1-Flash

tools/ui/src/lib/components/app/dialogs/DialogManageModels.svelte

@@ -136,7 +136,7 @@
 							class="gap-1.5"
 							onclick={() => (view = 'discover')}
 							size="sm"
-							variant="secondary"
+							variant="tertiary"
 						>
 							<Compass class="h-3.5 w-3.5" />
 
@@ -147,7 +147,7 @@
 							class="gap-1.5"
 							onclick={() => (view = 'providers')}
 							size="sm"
-							variant="secondary"
+							variant="tertiary"
 						>
 							<Server class="h-3.5 w-3.5" />
 

tools/ui/src/lib/components/ui/button/button.svelte

@@ -27,6 +27,8 @@
 				outline:
 					'shadow-sm hover:text-accent-foreground hover:bg-muted-foreground/10 backdrop-blur-sm dark:border-input border',
 				secondary:
+					'bg-muted/30 dark:bg-muted-foreground/15 dark:text-secondary-foreground shadow-sm border-muted border text-foreground hover:bg-muted dark:hover:bg-muted-foreground/25',
+				tertiary:
 					'bg-muted/60 dark:bg-muted/75 shadow-sm border border-border/30 text-foreground hover:bg-muted/80 dark:border-border/20 dark:hover:bg-muted'
 			}
 		}