feat: mobile repo workspace with tabbed navigation

- Add mobile viewport detection to RepoWorkspace
- Implement bottom tab navigation (Files, Editor, Git, Terminal)
- Add repository and branch selectors for mobile
- Create mobile workspace layout with tab bar
- Add CSS styles for mobile workspace components
- Desktop layout remains unchanged

apps/web/src/styles.css

@@ -373,3 +373,73 @@
     display: none;
   }
 }
+
+/* Mobile Workspace */
+.mobile-workspace {
+  display: flex;
+  flex-direction: column;
+  height: calc(100vh - 4rem);
+  overflow: hidden;
+}
+
+.mobile-workspace-header {
+  display: flex;
+  gap: var(--space-2);
+  padding: var(--space-2);
+  border-bottom: 1px solid var(--border);
+  background: var(--surface);
+}
+
+.mobile-repo-selector,
+.mobile-branch-selector {
+  flex: 1;
+  padding: var(--space-2);
+  border: 1px solid var(--border);
+  border-radius: 6px;
+  background: var(--surface);
+  color: var(--text);
+  font-size: 0.875rem;
+  min-height: 40px;
+}
+
+.mobile-workspace-content {
+  flex: 1;
+  overflow-y: auto;
+  overflow-x: hidden;
+}
+
+.mobile-workspace-tabs {
+  display: flex;
+  border-top: 1px solid var(--border);
+  background: var(--surface);
+  padding-bottom: env(safe-area-inset-bottom);
+}
+
+.mobile-workspace-tab {
+  flex: 1;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  gap: 2px;
+  padding: var(--space-2) var(--space-1);
+  border: none;
+  background: transparent;
+  color: var(--muted);
+  font-size: 0.625rem;
+  cursor: pointer;
+  transition: color 0.15s ease;
+  min-height: 56px;
+}
+
+.mobile-workspace-tab.active {
+  color: var(--brand);
+}
+
+.mobile-workspace-tab:active {
+  opacity: 0.7;
+}
+
+.mobile-git-view {
+  padding: var(--space-2);
+}