test(desktop): e2e proving session-load re-render bug

Adds an E2E test that reproduces the multi-pass transcript re-render
bug when loading a large previous session. The desktop's resumeSession
path does up to three message-set passes (warm cache → session.activate
projection → REST persisted transcript), each replacing the $messages
atom and rebuilding the transcript DOM — visible as a flicker on large
sessions.

The test:
- Seeds a real 53-message session (exported from state.db, stripped to
  minimal stubs — 13.5KB) into a sandbox HERMES_HOME via
  SessionDB.import_sessions before launch
- Opens the session from the sidebar
- Instruments the thread viewport with a MutationObserver to count DOM
  mutation bursts (groups separated by a 30ms gap)
- Asserts >= 2 bursts, proving the transcript was rebuilt more than once

Also exports createSandbox, writeMockProviderConfig, writeEnvFile,
buildAppEnv, findElectron, and launchDesktop from fixtures.ts so the
new spec can compose its own seeded-backend fixture without duplicating
the sandbox/config/launch logic.

apps/desktop/e2e/scripts/seed_session_db.py

@@ -0,0 +1,56 @@
+#!/usr/bin/env python3
+"""Seed a Hermes state.db with a session exported from a real conversation.
+
+Usage: seed_session_db.py <state_db_path> <fixture_json_path>
+
+Creates the database with the full SessionDB schema (if it doesn't exist)
+and imports the session from the JSON fixture. Uses the real
+SessionDB.import_sessions() so the data shape matches what the desktop
+backend expects.
+"""
+import json
+import sys
+from pathlib import Path
+
+# Add the repo root to sys.path so we can import hermes_state.
+# The script is invoked from apps/desktop/e2e/ — repo root is ../../..
+repo_root = Path(__file__).resolve().parents[3]
+sys.path.insert(0, str(repo_root))
+
+from hermes_state import SessionDB  # noqa: E402
+
+
+def main():
+    if len(sys.argv) != 3:
+        print(f"Usage: {sys.argv[0]} <state_db_path> <fixture_json_path>", file=sys.stderr)
+        sys.exit(1)
+
+    db_path = Path(sys.argv[1])
+    fixture_path = Path(sys.argv[2])
+
+    db_path.parent.mkdir(parents=True, exist_ok=True)
+
+    with open(fixture_path, "r", encoding="utf-8") as f:
+        session_data = json.load(f)
+
+    db = SessionDB(db_path=db_path)
+    result = db.import_sessions([session_data])
+
+    if not result.get("ok"):
+        print(f"Import failed: {result}", file=sys.stderr)
+        sys.exit(1)
+
+    imported = result.get("imported", 0)
+    skipped = result.get("skipped", 0)
+    errors = result.get("errors", [])
+
+    if errors:
+        print(f"Import had errors: {errors}", file=sys.stderr)
+        sys.exit(1)
+
+    print(f"Seeded {imported} session(s), skipped {skipped} → {db_path}")
+    db.close()
+
+
+if __name__ == "__main__":
+    main()