feat(settings): a Pet page for the mascot

Settings -> Pet (/settings/pet, PetSettings.tsx): show/hide, speech
bubbles, size with a live preview, and the pet grid - Sagi, featured
codex-pets.net pets (downloaded on first pick), your own (deletable) -
with image upload and .codex-pet.zip import. The Sagi block leaves
General; the mascot menu's Pet settings opens the page. PetMenu and the
page share components/Sagi/usePets.ts.

client/src/components/Sagi/Sagi.tsx

@@ -295,7 +295,7 @@ export function Sagi() {
             onClose={() => setMenu(false)}
             onSettings={() => {
               setMenu(false);
-              navigate("/settings/general#sagi");
+              navigate("/settings/pet");
             }}
           />
         </SagiFlyout>