feat: polish send button — hidden until content, icon-circle, pop-in animation

- index.html: btnSend hidden by default (display:none), icon-only (upward
  arrow SVG, no text label), title attribute for accessibility

- style.css: new send-btn design — 34px circle, blue fill (#7cb9ff),
  subtle glow box-shadow, scale() hover/active for tactile feel,
  .send-btn.visible with @keyframes send-pop-in (scale+opacity spring
  using cubic-bezier(.34,1.56,.64,1) for a satisfying pop). Mobile
  override updated to preserve circle dimensions.

- ui.js: updateSendBtn() — shows button with pop-in animation when
  textarea has content OR files are attached and agent is not busy;
  hides instantly when content is cleared. Hooked into setBusy() and
  renderTray() so button state tracks all content sources correctly.

- boot.js: input event listener calls updateSendBtn() on every keystroke.

- messages.js: autoResize() calls updateSendBtn() so button disappears
  immediately after send clears the textarea.

- tests/test_sprint21.py: 33 tests covering HTML structure, CSS design
  (circle shape, colors, animations, keyframes), JS logic (updateSendBtn,
  setBusy, renderTray, autoResize integration), and regressions
  (363 total, all pass).

static/ui.js

@@ -187,9 +187,25 @@ function setStatus(t){
     if(dismiss)dismiss.style.display=(!transient && !S.busy)?'inline':'none';
   }
 }
+function updateSendBtn(){
+  const btn=$('btnSend');
+  if(!btn) return;
+  const hasContent=$('msg').value.trim().length>0||S.pendingFiles.length>0;
+  const shouldShow=hasContent&&!S.busy;
+  if(shouldShow&&btn.style.display==='none'){
+    btn.style.display='';
+    // Remove then re-add class to retrigger animation each time
+    btn.classList.remove('visible');
+    requestAnimationFrame(()=>btn.classList.add('visible'));
+  } else if(!shouldShow&&btn.style.display!=='none'){
+    btn.style.display='none';
+    btn.classList.remove('visible');
+  }
+}
 function setBusy(v){
   S.busy=v;
   $('btnSend').disabled=v;
+  updateSendBtn();
   const dots=$('activityDots');
   if(dots) dots.style.display=v?'flex':'none';
   if(!v){
@@ -920,8 +936,9 @@ async function promptNewFolder(){
 
 function renderTray(){
   const tray=$('attachTray');tray.innerHTML='';
-  if(!S.pendingFiles.length){tray.classList.remove('has-files');return;}
+  if(!S.pendingFiles.length){tray.classList.remove('has-files');updateSendBtn();return;}
   tray.classList.add('has-files');
+  updateSendBtn();
   S.pendingFiles.forEach((f,i)=>{
     const chip=document.createElement('div');chip.className='attach-chip';
     chip.innerHTML=`&#128206; ${esc(f.name)} <button title="Remove">&#10005;</button>`;