feat(web): align composer and activity to content width, icon cancel button

Move max-width constraint from .structured-rows up to .structured-main
so the activity banner and composer match the transcript width. Replace
text cancel button with a stop-sign icon inside a spinning ring. Drop
the accent border-top on composer focus.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/styles.css

@@ -2290,6 +2290,9 @@ input.leaf-rename:focus {
 .structured-main {
   min-width: 0;
   min-height: 0;
+  max-width: 820px;
+  margin-inline: auto;
+  width: 100%;
   display: flex;
   flex-direction: column;
   flex: 1 1 auto;
@@ -2306,9 +2309,6 @@ input.leaf-rename:focus {
   display: flex;
   flex-direction: column;
   min-width: 0;
-  max-width: 820px;
-  margin-inline: auto;
-  width: 100%;
 }
 
 .structured-activity {
@@ -3104,7 +3104,7 @@ input.leaf-rename:focus {
 }
 
 .structured-composer:focus-within {
-  border-top-color: var(--accent);
+  border-top-color: var(--border);
 }
 
 .structured-input-wrap {
@@ -3220,7 +3220,9 @@ input.leaf-rename:focus {
 }
 
 .structured-composer .structured-cancel {
-  padding: 0 var(--sp-3);
+  position: relative;
+  width: 40px;
+  padding: 0;
   border-color: color-mix(in oklab, var(--danger) 35%, var(--border));
   color: var(--danger);
 }
@@ -3229,6 +3231,23 @@ input.leaf-rename:focus {
   background: var(--danger-tint);
 }
 
+.structured-cancel-stop {
+  width: 16px;
+  height: 16px;
+}
+
+.structured-cancel-spinner {
+  position: absolute;
+  inset: 4px;
+  width: auto;
+  height: auto;
+  animation: cancel-spin 0.8s linear infinite;
+}
+
+@keyframes cancel-spin {
+  to { transform: rotate(360deg); }
+}
+
 @media (max-width: 520px) {
   /* One row cannot hold a draft plus three controls at phone width — the
      draft gets a line of its own and the controls drop below it. */