ui: fix group checkbox sticking checked after disable

Clicking a mixed-state group box let bits-ui optimistically flip
its internal checked flag; the derived checked prop did not change
across the transition (both mixed and off map to checked=false),
so Svelte never applied the settled value and the check icon stuck
while the count already read 0/7.

Pass the parent flag as checked and the mix as indeterminate, so
every group toggle changes checked; render the dash on top of a
checked box for the mixed state.

Assisted-by: pi

tools/ui/src/lib/components/app/chat/ChatForm/ChatFormActions/ChatFormActionAdd/ChatFormActionAddSheet.svelte

@@ -276,7 +276,7 @@
 		</span>
 
 		<Checkbox
-			checked={checkState.checked && !checkState.indeterminate}
+			checked={checkState.checked}
 			class="{ICON_CLASS_DEFAULT} shrink-0"
 			indeterminate={checkState.indeterminate}
 			onCheckedChange={() => toolsPanel.toggleGroupByKey(group.key)}

tools/ui/src/lib/components/app/chat/ChatForm/ChatFormActions/ChatFormActionAdd/ChatFormActionAddToolsSubmenu.svelte

@@ -120,7 +120,7 @@
 					{#snippet child({ props })}
 						<Checkbox
 							{...props}
-							checked={checkState.checked && !checkState.indeterminate}
+							checked={checkState.checked}
 							class="mr-2 {ICON_CLASS_DEFAULT} shrink-0"
 							indeterminate={checkState.indeterminate}
 							onCheckedChange={() => toolsPanel.toggleGroupByKey(group.key)}

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

@@ -26,10 +26,10 @@
 >
 	{#snippet children({ checked, indeterminate })}
 		<div class="text-current transition-none" data-slot="checkbox-indicator">
-			{#if checked}
-				<CheckIcon class="size-3.5" />
-			{:else if indeterminate}
+			{#if indeterminate}
 				<MinusIcon class="size-3.5" />
+			{:else if checked}
+				<CheckIcon class="size-3.5" />
 			{/if}
 		</div>
 	{/snippet}