feat(FN-803): add ahead/behind commit lists to Git Manager Remotes tab

- Add backend API routes for remote commit lists (local-ahead, remote-ahead, all commits by branch)
- Surface ahead/behind commit counts and expandable commit lists in Remotes tab UI
- Add expand/collapse rows showing commit hash, message, author, and relative date
- Style commit list rows with alternating backgrounds and monospace hashes
- Add comprehensive tests for API routes and GitManagerModal component
- Update README documentation for Git Manager Remotes tab

packages/dashboard/app/styles.css

@@ -13445,6 +13445,111 @@ html .column.drag-over * {
   padding: var(--space-xs);
 }
 
+.gm-remote-item.selected {
+  border-color: var(--primary);
+  background: var(--primary-bg, color-mix(in srgb, var(--primary) 8%, transparent));
+}
+
+.gm-commits-to-push {
+  padding: var(--space-sm) 0;
+  display: flex;
+  flex-direction: column;
+  gap: var(--space-xs);
+}
+
+.gm-commits-to-push h5 {
+  font-size: 12px;
+  font-weight: 600;
+  color: var(--text-muted);
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+  margin: 0;
+}
+
+.gm-commits-count {
+  display: flex;
+  align-items: center;
+  gap: var(--space-xs);
+  font-size: 12px;
+  color: var(--text-muted);
+}
+
+.gm-commits-empty {
+  font-size: 12px;
+  color: var(--text-muted);
+  padding: var(--space-sm) 0;
+}
+
+.gm-commit-item-compact {
+  display: flex;
+  align-items: flex-start;
+  gap: var(--space-sm);
+  padding: var(--space-xs) var(--space-sm);
+  border-radius: var(--radius-sm);
+  font-size: 13px;
+}
+
+.gm-commit-item-compact:hover {
+  background: var(--hover);
+}
+
+.gm-commit-compact-hash {
+  flex-shrink: 0;
+  font-family: var(--font-mono);
+  font-size: 12px;
+  color: var(--text-muted);
+  padding-top: 1px;
+}
+
+.gm-commit-compact-info {
+  flex: 1;
+  min-width: 0;
+  display: flex;
+  flex-direction: column;
+  gap: 2px;
+}
+
+.gm-commit-compact-info .gm-commit-message {
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+  color: var(--text);
+}
+
+.gm-commit-compact-info .gm-commit-meta {
+  display: flex;
+  align-items: center;
+  gap: var(--space-xs);
+  font-size: 11px;
+  color: var(--text-muted);
+}
+
+.gm-remote-commits-section {
+  margin-top: var(--space-md);
+  border-top: 1px solid var(--border);
+  padding-top: var(--space-md);
+}
+
+.gm-remote-commits-section .gm-section-header {
+  margin-bottom: var(--space-sm);
+}
+
+.gm-remote-commits-section h5 {
+  font-size: 12px;
+  font-weight: 600;
+  color: var(--text-muted);
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+  margin: 0 0 var(--space-sm) 0;
+}
+
+.gm-remote-commits-list {
+  max-height: 280px;
+  overflow-y: auto;
+  display: flex;
+  flex-direction: column;
+}
+
 /* ── Responsive ── */
 
 @media (max-width: 640px) {