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/index.html

@@ -264,6 +264,10 @@
               </svg>
             </button>
           </div>
+          <div class="ctx-indicator" id="ctxIndicator" style="display:none" title="Context window usage">
+            <span class="ctx-bar-wrap"><span class="ctx-bar" id="ctxBar"></span></span>
+            <span class="ctx-label" id="ctxLabel"></span>
+          </div>
           <div class="composer-right">
             <button class="send-btn" id="btnSend" title="Send message" style="display:none">
               <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2.5" stroke-linecap="round" stroke-linejoin="round"><line x1="12" y1="19" x2="12" y2="5"/><polyline points="5 12 12 5 19 12"/></svg>

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 {

static/style.css

@@ -183,6 +183,13 @@
   textarea#msg::placeholder{color:var(--muted);}
   .composer-footer{display:flex;align-items:center;justify-content:space-between;padding:6px 10px 10px;}
   .composer-left{display:flex;gap:2px;align-items:center;}
+  /* Context usage indicator */
+  .ctx-indicator{display:flex;align-items:center;gap:6px;padding:2px 4px;flex-shrink:1;min-width:0;}
+  .ctx-bar-wrap{width:70px;height:5px;border-radius:3px;background:rgba(255,255,255,.08);overflow:hidden;flex-shrink:0;}
+  .ctx-bar{display:block;height:100%;border-radius:3px;transition:width .4s ease,background .4s ease;min-width:2px;background:var(--teal);}
+  .ctx-bar.ctx-mid{background:#e6a817;}
+  .ctx-bar.ctx-high{background:#e05252;}
+  .ctx-label{font-size:9px;color:var(--muted);white-space:nowrap;font-variant-numeric:tabular-nums;}
   .composer-right{display:flex;gap:6px;align-items:center;}
   .icon-btn{width:34px;height:34px;border-radius:8px;background:none;border:none;color:var(--muted);cursor:pointer;display:flex;align-items:center;justify-content:center;font-size:16px;transition:all .15s;}
   .icon-btn{opacity:.75;}

static/ui.js

@@ -84,6 +84,32 @@ let _scrollPinned=true;
 })();
 function _fmtTokens(n){if(!n||n<0)return'0';if(n>=1e6)return(n/1e6).toFixed(1)+'M';if(n>=1e3)return(n/1e3).toFixed(1)+'k';return String(n);}
 
+// Context usage indicator in composer footer
+function _syncCtxIndicator(usage){
+  const el=$('ctxIndicator');
+  if(!el)return;
+  const inTok=usage.input_tokens||0;
+  const outTok=usage.output_tokens||0;
+  const total=inTok+outTok;
+  if(!total){el.style.display='none';return;}
+  el.style.display='';
+  // Estimate context window fill (assume 128k default, common for most models)
+  const ctxWindow=128000;
+  const pct=Math.min(100,Math.round((inTok/ctxWindow)*100));
+  const bar=$('ctxBar');
+  const label=$('ctxLabel');
+  if(bar){
+    bar.style.width=pct+'%';
+    bar.className='ctx-bar'+(pct>75?' ctx-high':pct>50?' ctx-mid':'');
+  }
+  if(label){
+    const cost=usage.estimated_cost;
+    let text=`${_fmtTokens(inTok)} in \u00b7 ${_fmtTokens(outTok)} out`;
+    if(cost) text+=` \u00b7 $${cost<0.01?cost.toFixed(4):cost.toFixed(2)}`;
+    label.textContent=text;
+  }
+}
+
 function scrollIfPinned(){
   if(!_scrollPinned) return;
   const el=$('messages');