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/store/sessionStore.test.ts

@@ -54,6 +54,30 @@ test("info events accumulate in stepMessages", () => {
   ]);
 });
 
+test("an error-level info event flags the current phase", () => {
+  const st = useSessionStore.getState();
+  st.applyEvent({ type: "ui_request", ui_request: { id: "u", widget: "select", phase: "F4_schema_linking" } });
+  st.applyEvent({ type: "info", level: "error", text: "boom" });
+  expect(useSessionStore.getState().phaseError).toBe("F4");
+  // the message still lands in stepMessages
+  expect(useSessionStore.getState().stepMessages.at(-1)).toEqual({ level: "error", text: "boom" });
+});
+
+test("a non-error info event does not set phaseError", () => {
+  const st = useSessionStore.getState();
+  st.applyEvent({ type: "ui_request", ui_request: { id: "u", widget: "select", phase: "F4_x" } });
+  st.applyEvent({ type: "info", level: "warning", text: "w" });
+  expect(useSessionStore.getState().phaseError).toBeNull();
+});
+
+test("the next ui_request clears phaseError", () => {
+  const st = useSessionStore.getState();
+  st.applyEvent({ type: "ui_request", ui_request: { id: "u", widget: "select", phase: "F4_x" } });
+  st.applyEvent({ type: "info", level: "error", text: "boom" });
+  st.applyEvent({ type: "ui_request", ui_request: { id: "u2", widget: "select", phase: "F5_x" } });
+  expect(useSessionStore.getState().phaseError).toBeNull();
+});
+
 test("setLastUserEntry records the entry and clears stepMessages", () => {
   const st = useSessionStore.getState();
   st.applyEvent({ type: "info", level: "info", text: "x" });