refactor: extract CSS modules for session/settings and delete styles.css (Task 2.3)

- Create InstanceList.module.css, AppShell.module.css, SettingsTabLayout.module.css
- Create CommitPanel.module.css, FileViewer.module.css
- Create page CSS files: sessions, repo-workspace, dashboard, projects,
  git-history, ssh-keys, settings
- Update components to import and use CSS modules
- Delete monolithic styles.css (2,255 lines)
- Update main.tsx to import page CSS and new modules

Quality gates: tsc (pass), eslint (pass), build (pass)
Refs: repo-restructure Task 2.3

apps/web/src/components/features/settings/SettingsTabLayout.module.css

@@ -0,0 +1,89 @@
+.settingsLayout {
+  display: flex;
+  gap: 2rem;
+  padding: 1.5rem 0;
+}
+
+.settingsSidebar {
+  width: 200px;
+  flex-shrink: 0;
+}
+
+.settingsNav {
+  display: flex;
+  flex-direction: column;
+  gap: 0.25rem;
+}
+
+.settingsNavLink {
+  padding: 0.625rem 1rem;
+  border-radius: 8px;
+  color: var(--muted);
+  text-decoration: none;
+  font-size: 0.95rem;
+  transition: all 0.2s;
+}
+
+.settingsNavLink:hover {
+  background: var(--bg);
+  color: var(--ink);
+}
+
+.settingsNavLinkActive {
+  background: var(--brand);
+  color: white;
+  font-weight: 500;
+}
+
+.settingsContent {
+  flex: 1;
+  min-width: 0;
+}
+
+.settingsPanel {
+  background: var(--panel);
+  border: 1px solid var(--border);
+  border-radius: 14px;
+  padding: 1.5rem;
+}
+
+.settingsBreadcrumb {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  font-size: 0.9rem;
+  color: var(--muted);
+  margin-bottom: 1rem;
+}
+
+.settingsBreadcrumb a {
+  color: var(--brand);
+  text-decoration: none;
+}
+
+.settingsBreadcrumb a:hover {
+  text-decoration: underline;
+}
+
+@media (max-width: 768px) {
+  .settingsLayout {
+    flex-direction: column;
+    gap: 1rem;
+  }
+
+  .settingsSidebar {
+    width: 100%;
+  }
+
+  .settingsNav {
+    flex-direction: row;
+    overflow-x: auto;
+    padding-bottom: 0.5rem;
+  }
+
+  .settingsNavLink {
+    white-space: nowrap;
+    padding: 0.5rem 0.75rem;
+    font-size: 0.875rem;
+  }
+}