feat: add MergeDialog component and integrate with GitToolbar

- Create MergeDialog component for branch merging
- Add merge button to GitToolbar
- Show source branch selector and target branch display
- Handle merge success and error states
- Add modal CSS styles

Completes Task 6.2 of git-control change.

apps/web/src/styles.css

@@ -950,3 +950,87 @@ a {
   color: #ef4444;
   font-size: 0.8125rem;
 }
+
+/* Modal Styles */
+.modal-overlay {
+  position: fixed;
+  top: 0;
+  left: 0;
+  right: 0;
+  bottom: 0;
+  background: rgba(0, 0, 0, 0.5);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  z-index: 1000;
+}
+
+.modal-content {
+  background: var(--panel);
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  padding: 1.5rem;
+  min-width: 400px;
+  max-width: 600px;
+  max-height: 80vh;
+  overflow: auto;
+}
+
+.modal-content h2 {
+  margin: 0 0 1rem 0;
+  font-size: 1.25rem;
+}
+
+.modal-actions {
+  display: flex;
+  justify-content: flex-end;
+  gap: 0.75rem;
+  margin-top: 1.5rem;
+}
+
+/* Merge Dialog */
+.merge-form {
+  display: flex;
+  flex-direction: column;
+  gap: 1rem;
+}
+
+.merge-form .form-field {
+  display: flex;
+  flex-direction: column;
+  gap: 0.375rem;
+}
+
+.merge-form label {
+  font-size: 0.875rem;
+  font-weight: 500;
+}
+
+.merge-form select,
+.merge-form input,
+.merge-form textarea {
+  padding: 0.5rem;
+  border: 1px solid var(--border);
+  border-radius: 4px;
+  background: var(--bg);
+  color: var(--ink);
+  font-family: inherit;
+  font-size: 0.875rem;
+}
+
+.merge-form textarea {
+  resize: vertical;
+}
+
+.input-disabled {
+  opacity: 0.6;
+  cursor: not-allowed;
+}
+
+.success-text {
+  color: #10b981;
+  font-size: 0.875rem;
+  padding: 0.5rem;
+  background: rgba(16, 185, 129, 0.1);
+  border-radius: 4px;
+}