chore: remove committed session cookies, debug artifacts and dead code

Repo hygiene pass:
- cookie.txt / cookies.txt held real session cookies and must never be
  committed; .gitignore now blocks them along with .env files
- half a megabyte of debug screenshots, one-shot production hotfix scripts
  (all superseded by the automatic startup migrations), scratch files and
  the unused attached_assets folder (with its dangling @assets vite alias)
- dead code: server/storage-old.ts (unreferenced, 167 of the project's 430
  TypeScript errors) and five client pages no route ever imported
  (BLReconciliationNative, Avoirs_backup, TasksSimplified, TasksListSimple,
  TasksProductionSimple)

TypeScript error count drops from 430 to 261 with no behavior change; the
client build is unaffected.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FHdE9nEh8zHhQNGpCgrtYe

.gitignore

@@ -3,4 +3,13 @@ dist
 .DS_Store
 server/public
 vite.config.ts.*
-*.tar.gz
+*.tar.gz
+
+# Secrets et fichiers de session : ne jamais committer
+.env
+.env.local
+cookie*.txt
+
+# Artefacts de debug locaux
+*.log
+scratch/