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>