style(home): use neutral gray "New note" button instead of blue primary

The button rendered with Mantine's default blue primary, which clashes
with the app's neutral/dark accent design. Switch both the single-space
and the space-picker variants to variant="light" color="gray".

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

apps/client/src/features/home/components/new-note-button.tsx

@@ -55,6 +55,8 @@ export default function NewNoteButton() {
       <Button
         fullWidth
         size="md"
+        variant="light"
+        color="gray"
         leftSection={<IconPlus size={18} />}
         loading={isPending}
         onClick={() => createNote(writableSpaces[0])}
@@ -71,6 +73,8 @@ export default function NewNoteButton() {
         <Button
           fullWidth
           size="md"
+          variant="light"
+          color="gray"
           leftSection={<IconPlus size={18} />}
           loading={isPending}
         >