feat(nofxi): full i18n bilingual support (zh/en)

Agent i18n:
- All command responses bilingual (help, status, positions, balance, trades, analysis, settings, errors)
- Language auto-detected from web UI lang toggle
- Stored per-user in SQLite preferences
- AI chat system prompt switches to Chinese/English based on user lang
- Trade confirmation prompts bilingual

Web UI:
- Sends lang parameter with every API request
- Agent extracts [lang:xx] prefix and persists preference

i18n message catalog: nofxi/internal/agent/i18n.go

nofxi/web/index.html

@@ -337,7 +337,7 @@ async function send(text){
   addM('user',text);
   typEl.classList.add('on');sBtnEl.disabled=true;inpEl.focus();
   try{
-    const r=await fetch(API+'/api/chat',{method:'POST',headers:{'Content-Type':'application/json'},body:JSON.stringify({message:text,user_id:1})});
+    const r=await fetch(API+'/api/chat',{method:'POST',headers:{'Content-Type':'application/json'},body:JSON.stringify({message:text,user_id:1,lang:lang})});
     const d=await r.json();
     addM('bot',d.response||d.error||'No response');
   }catch(e){addM('bot','⚠️ Error: '+e.message)}