Is there a window where you can react before « assault » or « shield » resolve?

No, there is no window to react to Assault or Shield because they do not use the chain.

According to Rule 807.1 and Rule 814.1, Assault and Shield are Passive Ability keywords. Passive abilities function continuously and do not use the chain (as they do not create triggered abilities that enter the chain, nor are they cards or activated abilities).

Because they are passive, they are active as long as the unit has the corresponding "Attacker" or "Defender" designation, and there is no point in the game where these abilities are placed on the chain for you to respond to.

Key points to remember:

• Passive vs. Triggered: Do not confuse these keywords with "When I attack" or "When I defend" triggered abilities. While those triggers do open an initial chain (which you can react to), Assault and Shield themselves are passive modifiers that apply automatically when the combat designations are established.
• No Window: Since they never enter the chain, they offer no "window of opportunity" to play reactions or actions in response to them.