feat: context usage indicator in composer footer

Shows a compact bar + label in the composer footer after the first
response, displaying input/output token counts, context window fill
percentage, and estimated cost. Bar turns yellow >50% and red >75%.

Updates on every response completion via the existing usage data from
the done SSE event. Hidden until first response (no usage data yet).

Inspired by PR #75 (@MartinNielsenDev).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/messages.js

@@ -149,7 +149,7 @@ async function send(){
         // Stamp _ts on the last assistant message if it has no timestamp
         const lastAsst=[...S.messages].reverse().find(m=>m.role==='assistant');
         if(lastAsst&&!lastAsst._ts&&!lastAsst.timestamp) lastAsst._ts=Date.now()/1000;
-        if(d.usage) S.lastUsage=d.usage;
+        if(d.usage){S.lastUsage=d.usage;_syncCtxIndicator(d.usage);}
         if(d.session.tool_calls&&d.session.tool_calls.length){
           S.toolCalls=d.session.tool_calls.map(tc=>({...tc,done:true}));
         } else {