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/components/Layout.tsx

@@ -1,7 +1,7 @@
 /**
  * @file Layout.tsx
  * @description Application shell that frames every authenticated route: persistent
- * sidebar, main content column, update notifier, and the Tabby assistant overlay.
+ * sidebar, main content column, update notifier, and the Sagi assistant overlay.
  * The layout is the single parent route in {@link App} — child pages render inside
  * React Router's `<Outlet />` so navigation never remounts chrome.
  *
@@ -38,7 +38,7 @@
  * ## Internal dependencies
  * - `./Sidebar`
  * - `./UpdateNotifier`
- * - `./Tabby/Tabby`
+ * - `./Sagi/Sagi`
  *
  * ## Public surface
  * - `Layout` — exported API; see TSDoc on the symbol for behavior.
@@ -68,7 +68,7 @@ import { Outlet } from "react-router-dom";
 import { useTranslation } from "react-i18next";
 import { Sidebar, SIDEBAR_STORAGE_KEY, loadCollapsed } from "./Sidebar";
 import { UpdateNotifier } from "./UpdateNotifier";
-import { Tabby } from "./Tabby/Tabby";
+import { Sagi } from "./Sagi/Sagi";
 
 /** Props for {@link Layout}. */
 interface LayoutProps {
@@ -100,7 +100,7 @@ export function Layout({ wsConnected }: LayoutProps) {
         {t("skipToContent")}
       </a>
       <UpdateNotifier />
-      <Tabby />
+      <Sagi />
       <Sidebar wsConnected={wsConnected} collapsed={collapsed} onToggle={toggle} />
       <main
         id="main-content"