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>

server/shared/utils.ts

@@ -346,6 +346,43 @@ export function createNormalizedMessage(fields: NormalizedMessageInput): Normali
   };
 }
 
+/**
+ * Build the unified terminal `complete` lifecycle message.
+ *
+ * Contract: every provider run ends with exactly one `complete` (the
+ * abort-session handler emits it on behalf of cancelled runs, so aborted runs
+ * must NOT emit their own). The frontend treats `complete` as the only
+ * terminal signal and never needs provider-specific handling:
+ *
+ * - `sessionId`     — the id the client knows this run by ('' if never discovered)
+ * - `actualSessionId` — canonical id after the run; equals `sessionId` unless
+ *                       the provider rewrote it mid-run
+ * - `exitCode`      — 0 on success; a missing/null code (e.g. killed process)
+ *                     is reported as failure
+ * - `success`       — exitCode === 0 and not aborted
+ * - `aborted`       — run was cancelled by the user
+ */
+export function createCompleteMessage(opts: {
+  provider: NormalizedMessage['provider'];
+  sessionId?: string | null;
+  actualSessionId?: string | null;
+  exitCode?: number | null;
+  aborted?: boolean;
+}): NormalizedMessage {
+  const exitCode = typeof opts.exitCode === 'number' ? opts.exitCode : 1;
+  const aborted = Boolean(opts.aborted);
+
+  return createNormalizedMessage({
+    kind: 'complete',
+    provider: opts.provider,
+    sessionId: opts.sessionId || null,
+    actualSessionId: opts.actualSessionId || opts.sessionId || null,
+    exitCode,
+    success: exitCode === 0 && !aborted,
+    aborted,
+  });
+}
+
 // ---------------------------
 //----------------- MCP CONFIG PARSING UTILITIES ------------
 /**