feat(frontend): phase progress dots (B) + compact sidebar redesign (C)

B — WorkflowBar renders F1..F8 as colored ring-dots (no phase-name text):
green=done, amber=running (subtle pulse), red=error, gray=pending; green
connectors lead the active dot, each dot carries data-state. Lightweight
error signal: sessionStore gains `phaseError`, set when an info event has
level=error during the active phase, cleared on the next ui_request.

C — denser single-line session rows (inline status dot + name, py-1), a
3-level type hierarchy (L1 SESSIONS / L2 section+group headers / L3 names),
and the "No group" label removed (ungrouped render after the last group,
guarded so the empty-state still teaches when there are zero groups).

Live-verified with Playwright (all four dot states, sidebar hierarchy, and
E's deferred activity-panel check). Frontend 93/93, tsc -b clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/shell/AppShell.session-mgmt.test.tsx

@@ -36,6 +36,19 @@ test("active list shows group header and hides archived sessions", async () => {
   expect(screen.queryByText("Archiviata due")).not.toBeInTheDocument(); // archived hidden
 });
 
+test("ungrouped sessions render after groups with no 'No group' label", async () => {
+  server.use(
+    http.get("http://localhost:8787/sessions", () => HttpResponse.json([
+      { id: "g1", status: "open", question: "In gruppo", summary: null, created_at: "2026-01-02T00:00:00Z", updated_at: null, author: null, name: null, group: "Aritmologia", archived: false },
+      { id: "u1", status: "open", question: "Senza gruppo", summary: null, created_at: "2026-01-03T00:00:00Z", updated_at: null, author: null, name: null, group: null, archived: false },
+    ])),
+  );
+  wrap();
+  expect(await screen.findByText("In gruppo")).toBeInTheDocument(); // grouped
+  expect(screen.getByText("Senza gruppo")).toBeInTheDocument(); // ungrouped, still listed
+  expect(screen.queryByText(/no group/i)).not.toBeInTheDocument();
+});
+
 test("opening the panel shows the session documents", async () => {
   wrap();
   await userEvent.click(await screen.findByText("Attiva uno"));