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.ts

@@ -14,6 +14,7 @@ interface SessionState {
   lastUserEntry: { kind: "input" | "choice"; text: string } | null;
   lastSystemEvent: StreamEvent | null;
   currentPhase: string | null;
+  phaseError: string | null;
   applyEvent: (e: StreamEvent) => void;
   clearPending: () => void;
   resetSession: () => void;
@@ -30,6 +31,7 @@ const empty = {
   lastUserEntry: null as { kind: "input" | "choice"; text: string } | null,
   lastSystemEvent: null,
   currentPhase: null as string | null,
+  phaseError: null as string | null,
 };
 
 export const useSessionStore = create<SessionState>((set) => ({
@@ -42,6 +44,8 @@ export const useSessionStore = create<SessionState>((set) => ({
           currentPhase: e.ui_request.phase
             ? e.ui_request.phase.split("_")[0]
             : st.currentPhase,
+          // A new gate means the phase moved on (or re-presented): clear any error flag.
+          phaseError: null,
         };
       if (e.type === "text_delta") {
         const t = [...st.transcript];
@@ -50,7 +54,11 @@ export const useSessionStore = create<SessionState>((set) => ({
         else t.push({ role: "assistant", text: e.text });
         return { transcript: t };
       }
-      if (e.type === "info") return { stepMessages: [...st.stepMessages, { level: e.level ?? "info", text: e.text }] };
+      if (e.type === "info") {
+        const stepMessages = [...st.stepMessages, { level: e.level ?? "info", text: e.text }];
+        // An error during the active phase marks that phase red until the next gate.
+        return e.level === "error" ? { stepMessages, phaseError: st.currentPhase } : { stepMessages };
+      }
       if (e.type === "system_event") return { lastSystemEvent: e };
       return {};
     }),