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/boot.js

@@ -145,6 +145,7 @@ $('modelSelect').onchange=async()=>{
 };
 $('msg').addEventListener('input',()=>{
   autoResize();
+  updateSendBtn();
   const text=$('msg').value;
   if(text.startsWith('/')&&text.indexOf('\n')===-1){
     const prefix=text.slice(1);