feat(companion): rename Tabby companion to Sagi, replace cat avatar with mascot

Full rename across files, identifiers, CSS classes, and user-facing strings
(en/vi locales, ARCHITECTURE.md). Replaces the hand-drawn animated cat
avatar with the pig-superhero mascot artwork, split into parts to keep
per-eye cursor tracking and blink animation working; whole-mascot mood
transforms (breathe/bob/shake/tilt) and new zzz/bang/sparkle overlays
carry the rest of the mood expression since the traced art has no shared
palette to key off of.

client/src/hooks/useWebSocket.ts

@@ -166,7 +166,7 @@ export function useWebSocket(onMessage: MessageHandler) {
   // back: tab refocus, regained network, or page becoming visible again. This
   // cancels any pending backoff timer and resets the attempt counter so we
   // don't sit out a long delay - e.g. after the dashboard server restarts, the
-  // socket (and the Tabby eyes) recover the moment you look at the tab.
+  // socket (and the Sagi eyes) recover the moment you look at the tab.
   useEffect(() => {
     const reconnectNow = () => {
       if (!mountedRef.current) return;