feat(frontend): add remove button to pending and submitted file cards

Lets users drop an uploaded file from the UI list before or after
conversion, without cancelling anything server-side.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/styles/home.css

@@ -215,6 +215,28 @@
   white-space: nowrap;
 }
 
+.file-remove-button {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  width: 28px;
+  height: 28px;
+  flex-shrink: 0;
+  border: none;
+  border-radius: 8px;
+  background: transparent;
+  color: var(--color-foreground);
+  opacity: 0.5;
+  cursor: pointer;
+  transition: opacity 150ms ease, background 150ms ease, color 150ms ease;
+}
+
+.file-remove-button:hover {
+  opacity: 1;
+  color: var(--color-destructive);
+  background: color-mix(in srgb, var(--color-destructive) 12%, transparent);
+}
+
 .file-config-controls {
   display: flex;
   flex-direction: column;