fix(attach): 附件改回浏览器下载通道，仅按钮触发且不再误报已开始

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/app/globals.css

@@ -1767,15 +1767,12 @@ body {
   text-decoration: none;
   color: inherit;
   font: inherit;
-  cursor: pointer;
+  cursor: default;
   border: 1px solid var(--line);
   background: var(--panel);
   box-shadow: var(--shadow-sm);
   transition: border-color 0.15s, box-shadow 0.15s, transform 0.15s, background 0.15s;
 }
-a.j13-att-card:hover {
-  color: inherit;
-}
 .j13-att-card:hover {
   transform: translateY(-1px);
   box-shadow: var(--shadow);
@@ -1800,13 +1797,7 @@ a.j13-att-card:hover {
   border-color: color-mix(in srgb, var(--ok) 46%, var(--line));
 }
 .j13-att-card.is-busy {
-  cursor: wait;
-}
-.j13-att-card:disabled {
-  opacity: 0.58;
-  cursor: not-allowed;
-  transform: none;
-  box-shadow: var(--shadow-sm);
+  opacity: 0.92;
 }
 .j13-att-card.is-fresh {
   animation: j13-att-unlock 0.7s ease-out;
@@ -1923,11 +1914,25 @@ a.j13-att-card:hover {
   font-weight: 600;
   line-height: 1;
   flex-shrink: 0;
-  pointer-events: none;
+  cursor: pointer;
   border: 1px solid var(--line-2);
   background: var(--panel);
   color: var(--ink);
   white-space: nowrap;
+  font-family: inherit;
+  text-decoration: none;
+  appearance: none;
+}
+a.j13-att-cta:hover {
+  color: var(--ink);
+}
+.j13-att-cta:hover:not(:disabled):not(.is-primary) {
+  border-color: var(--accent);
+  color: var(--accent);
+}
+.j13-att-cta:disabled {
+  opacity: 0.5;
+  cursor: not-allowed;
 }
 .j13-att-cta.is-primary {
   border-color: transparent;
@@ -1936,6 +1941,10 @@ a.j13-att-card:hover {
   box-shadow: 0 1px 2px rgba(16, 24, 40, 0.12),
     0 2px 8px color-mix(in srgb, var(--accent) 26%, transparent);
 }
+.j13-att-cta.is-primary:hover:not(:disabled) {
+  background: var(--accent-hi);
+  color: var(--accent-on);
+}
 .j13-att-pay {
   display: flex;
   flex-direction: column;