fix: admin password permanent + Help/My Tasks content rendering

- main.py: lifespan insère toujours le hash de 'FlowDeck2026!' pour admin
- dashboard.py + my_tasks.py: content_html passe via {% block content %} (Jinja)
- Avant: content_html ignoré car base.html utilise des blocs, pas des variables
- 143 tests passent

app/main.py

@@ -31,9 +31,12 @@ async def lifespan(_app: FastAPI):
     init_db()
     init_webhook_tables()
     from app.db import get_conn
+    from app.password_utils import hash_password
+    admin_hash = hash_password("FlowDeck2026!")
     with get_conn() as conn:
         conn.execute(
-            "INSERT OR IGNORE INTO users (login, full_name, email, is_admin) VALUES ('admin', 'Admin', '', 1)"
+            "INSERT OR IGNORE INTO users (login, full_name, email, password_hash, is_admin) VALUES ('admin', 'Admin', '', ?, 1)",
+            (admin_hash,)
         )
         conn.commit()
     logger.info("FlowDeck v4.0.0 started on port %d", settings.app_port)