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/i18n/locales/en/settings.json

@@ -4,10 +4,10 @@
   "exportData": "Export Data",
   "acrossSessions": "Across all tracked sessions",
   "basedOnUsage": "Based on per-model token usage",
-  "tabby": {
-    "title": "Tabby companion",
-    "description": "A floating cat that reacts to your live sessions.",
-    "enable": "Show Tabby",
+  "sagi": {
+    "title": "Sagi companion",
+    "description": "A floating mascot that reacts to your live sessions.",
+    "enable": "Show Sagi",
     "enableDesc": "Display the corner companion across the dashboard (⌘B to open)"
   },
   "pricing": {