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/index.html

@@ -231,9 +231,8 @@
             </button>
           </div>
           <div class="composer-right">
-            <button class="send-btn" id="btnSend">
-              <svg width="14" height="14" viewBox="0 0 24 24" fill="currentColor"><path d="M2.01 21L23 12 2.01 3 2 10l15 2-15 2z"/></svg>
-              Send
+            <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>
             </button>
           </div>
         </div>