feat(chat): derive activity indicator from per-session state and unify provider lifecycle events

Replace the chat processing banner with a minimal activity indicator and
rebuild the state model underneath it. The old banner was driven by five
overlapping pieces of state (isLoading, canAbortSession, claudeStatus in the
chat, plus two app-level Sets updated in lockstep through four callbacks)
that had to be kept in sync imperatively. Because completion and status
events mutated the *viewed* session's flags regardless of which session they
belonged to, a background session finishing could hide the indicator for a
still-running session, returning to a finished session could briefly show a
stale banner, and a late status reply could override a newer request.

The fix is structural rather than patch-by-patch: a single
Map<sessionId, {statusText, canInterrupt, startedAt}> in useSessionProtection
is now the only source of truth for "this session is working". The indicator,
stop button, composer streaming state, and session protection are all derived
from the viewed session's entry on render, so there is no stale local copy to
restore or reset when switching sessions. A PENDING_SESSION_ID sentinel
covers the window before a new conversation receives its real session id.
Terminal events delete the entry atomically, which is why the indicator
disappears the instant the final chunk arrives. Stale check-session-status
replies are discarded via an ifStartedBefore guard (an idle reply older than
the entry's startedAt describes a previous request, not the current one).

The second half unifies the provider lifecycle contract, because the frontend
could not be made race-free while each provider terminated differently:

- cursor emitted complete twice per run (result line + process close), which
  double-played the completion sound and let a late close-complete clear a
  newer request's indicator
- aborts produced two completes (the abort-session reply plus the provider's
  own non-aborted one), so cancelling a run played the celebration sound
- codex omitted exitCode; others attached ad-hoc fields (resultText, isError,
  isNewSession) the client had to know about
- claude/codex failures ended with only an error event while gemini/cursor
  also emit kind:'error' for mid-run stderr noise, so 'error' was ambiguous
  between "the run died" and "a process wrote to stderr"

Every run now ends with exactly one complete built by createCompleteMessage()
({sessionId, actualSessionId, exitCode, success, aborted}); abort-session
sends it on behalf of cancelled runs and providers detect the abort and skip
their own. error is demoted to an informational row, so stderr noise no
longer kills the indicator mid-run, and the client celebrates only
success: true completes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/hooks/useProjectsState.ts

@@ -12,12 +12,14 @@ import type {
   ProjectsUpdatedMessage,
 } from '../types/app';
 
+import type { SessionActivityMap } from './useSessionProtection';
+
 type UseProjectsStateArgs = {
   sessionId?: string;
   navigate: NavigateFunction;
   latestMessage: AppSocketMessage | null;
   isMobile: boolean;
-  activeSessions: Set<string>;
+  activeSessions: SessionActivityMap;
 };
 
 type FetchProjectsOptions = {

src/hooks/useSessionProtection.ts

@@ -1,55 +1,103 @@
 import { useCallback, useState } from 'react';
 
+/**
+ * Map key for a request that is in flight before the provider has announced
+ * its real session id (a brand-new conversation). `session_created` migrates
+ * the entry to the concrete session id.
+ */
+export const PENDING_SESSION_ID = '__pending_session__';
+
+export interface SessionActivity {
+  /** Provider-supplied status line; null renders the default activity label. */
+  statusText: string | null;
+  canInterrupt: boolean;
+  /**
+   * When this request was first marked as processing (client clock). Drives
+   * the elapsed-time display and the stale `session-status` reply guard.
+   */
+  startedAt: number;
+}
+
+export type SessionActivityMap = ReadonlyMap<string, SessionActivity>;
+
+export type MarkSessionProcessing = (
+  sessionId?: string | null,
+  activity?: { statusText?: string | null; canInterrupt?: boolean },
+) => void;
+
+export type MarkSessionIdle = (
+  sessionId?: string | null,
+  opts?: { ifStartedBefore?: number },
+) => void;
+
+/**
+ * Single source of truth for which sessions are actively processing a
+ * request. Everything the chat UI shows (activity indicator, abort
+ * availability, status text) is derived from this map; terminal events
+ * (`complete`, `error`, abort, an authoritative idle status reply) delete the
+ * entry atomically. The map also drives session protection: project refreshes
+ * are suppressed for sessions that have an entry here.
+ */
 export function useSessionProtection() {
-  const [activeSessions, setActiveSessions] = useState<Set<string>>(new Set());
-  const [processingSessions, setProcessingSessions] = useState<Set<string>>(new Set());
+  const [processingSessions, setProcessingSessions] = useState<Map<string, SessionActivity>>(
+    new Map(),
+  );
 
-  const markSessionAsActive = useCallback((sessionId?: string | null) => {
-    if (!sessionId) {
-      return;
-    }
-
-    setActiveSessions((prev) => new Set([...prev, sessionId]));
-  }, []);
-
-  const markSessionAsInactive = useCallback((sessionId?: string | null) => {
-    if (!sessionId) {
-      return;
-    }
-
-    setActiveSessions((prev) => {
-      const next = new Set(prev);
-      next.delete(sessionId);
-      return next;
-    });
-  }, []);
-
-  const markSessionAsProcessing = useCallback((sessionId?: string | null) => {
-    if (!sessionId) {
-      return;
-    }
-
-    setProcessingSessions((prev) => new Set([...prev, sessionId]));
-  }, []);
-
-  const markSessionAsNotProcessing = useCallback((sessionId?: string | null) => {
+  const markSessionProcessing = useCallback<MarkSessionProcessing>((sessionId, activity) => {
     if (!sessionId) {
       return;
     }
 
     setProcessingSessions((prev) => {
-      const next = new Set(prev);
-      next.delete(sessionId);
-      return next;
+      const existing = prev.get(sessionId);
+      const next: SessionActivity = {
+        statusText:
+          activity?.statusText !== undefined ? activity.statusText : existing?.statusText ?? null,
+        canInterrupt: activity?.canInterrupt ?? existing?.canInterrupt ?? true,
+        startedAt: existing?.startedAt ?? Date.now(),
+      };
+
+      if (
+        existing
+        && existing.statusText === next.statusText
+        && existing.canInterrupt === next.canInterrupt
+      ) {
+        return prev;
+      }
+
+      const updated = new Map(prev);
+      updated.set(sessionId, next);
+      return updated;
+    });
+  }, []);
+
+  const markSessionIdle = useCallback<MarkSessionIdle>((sessionId, opts) => {
+    if (!sessionId) {
+      return;
+    }
+
+    setProcessingSessions((prev) => {
+      const existing = prev.get(sessionId);
+      if (!existing) {
+        return prev;
+      }
+
+      // Guard against stale `check-session-status` replies: if a new request
+      // started after the check was sent, the idle reply describes the older
+      // request and must not clear the newer one.
+      if (opts?.ifStartedBefore !== undefined && existing.startedAt >= opts.ifStartedBefore) {
+        return prev;
+      }
+
+      const updated = new Map(prev);
+      updated.delete(sessionId);
+      return updated;
     });
   }, []);
 
   return {
-    activeSessions,
     processingSessions,
-    markSessionAsActive,
-    markSessionAsInactive,
-    markSessionAsProcessing,
-    markSessionAsNotProcessing,
+    markSessionProcessing,
+    markSessionIdle,
   };
 }