feat(FN-906): add file navigation to changes tab and reduce diff margins

- Add file navigation dropdown/keyboard support to the TaskChangesTab component
- Reduce horizontal margins in diff display for better readability
- Add comprehensive tests for file navigation behavior
- Include changeset for patch release

packages/dashboard/app/styles.css

@@ -3458,6 +3458,26 @@ body {
   margin: 0;
 }
 
+.changes-header-actions {
+  display: inline-flex;
+  align-items: center;
+  gap: var(--space-sm);
+}
+
+.changes-nav {
+  display: inline-flex;
+  align-items: center;
+  gap: var(--space-xs);
+}
+
+.changes-nav-indicator {
+  font-size: 12px;
+  color: var(--text-muted);
+  font-family: var(--font-mono);
+  min-width: 32px;
+  text-align: center;
+}
+
 .changes-file-list {
   display: flex;
   flex-direction: column;
@@ -3578,7 +3598,7 @@ body {
 
 .changes-diff-patch {
   margin: 0;
-  padding: var(--space-sm) var(--space-md);
+  padding: var(--space-sm) 0;
   background: rgba(0, 0, 0, 0.14);
   font-family: var(--font-mono);
   font-size: 11px;
@@ -15116,7 +15136,7 @@ html .column.drag-over * {
 
 .changes-diff-patch {
   margin: 0;
-  padding: var(--space-md);
+  padding: var(--space-sm) 0;
   font-size: 12px;
   line-height: 1.5;
   overflow-x: auto;