Add companion kitten — reactive corner mascot

A cozy bottom-right mascot that reacts to the writing session: cheers on
accepted suggestions and word-count milestones, drops Mandarin-first writing
tips, suggests screen breaks after long stretches, and naps when idle.

- useCompanion: library-agnostic behavior engine (priority + cooldown paced
  so it never nags); tips.ts holds all copy, bilingual zh-first.
- LottiePlayer wraps lottie-web's light build (offline, no eval/CDN) so it
  bundles into the Go binary. Ships with an emoji-kitten placeholder per mood;
  dropping a Lottie cat JSON into animations/index.ts is the only swap needed.
- Speech bubble uses the CJK-first font stack (spec Note #17).

Claude-Session: https://claude.ai/code/session_016Yr6jELuRc7hyzYLccQKZd

web/package-lock.json

@@ -15,6 +15,7 @@
         "@tiptap/pm": "^2.11.5",
         "@tiptap/react": "^2.11.5",
         "@tiptap/starter-kit": "^2.11.5",
+        "lottie-web": "^5.13.0",
         "react": "^19.1.0",
         "react-dom": "^19.1.0"
       },
@@ -2605,6 +2606,12 @@
         "uc.micro": "^2.0.0"
       }
     },
+    "node_modules/lottie-web": {
+      "version": "5.13.0",
+      "resolved": "https://registry.npmjs.org/lottie-web/-/lottie-web-5.13.0.tgz",
+      "integrity": "sha512-+gfBXl6sxXMPe8tKQm7qzLnUy5DUPJPKIyRHwtpCpyUEYjHYRJC/5gjUvdkuO2c3JllrPtHXH5UJJK8LRYl5yQ==",
+      "license": "MIT"
+    },
     "node_modules/lru-cache": {
       "version": "5.1.1",
       "resolved": "https://registry.npmjs.org/lru-cache/-/lru-cache-5.1.1.tgz",