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/messages.js

@@ -237,7 +237,7 @@ function transcript(){
   return lines.join('\n');
 }
 
-function autoResize(){const el=$('msg');el.style.height='auto';el.style.height=Math.min(el.scrollHeight,200)+'px';}
+function autoResize(){const el=$('msg');el.style.height='auto';el.style.height=Math.min(el.scrollHeight,200)+'px';updateSendBtn();}
 
 
 // ── Approval polling ──