feat: add in-browser ML demo with Transformers.js sentiment analysis

Adds a ~/demo page that runs DistilBERT sentiment analysis entirely in
the browser via ONNX Runtime Web + Transformers.js — no server required.

- New /demo route (lazy-loaded) with terminal-style UI
- Model download progress bar with status text
- Textarea input + 5 example texts to try
- Confidence bars (POSITIVE/NEGATIVE) with inference time display
- EN/ZH bilingual support integrated into existing i18n system
- vite.config: exclude @xenova/transformers from Vite pre-bundling

Author: Spectual