Convert `Modal`s with forms in them to `ModalForm`

[david-crespo]

Follow-up to #3192 and #3197. These modals use raw Modal + Modal.Footer + useApiMutation with onError → addToast. Converting to ModalForm gives them inline submit errors and a loading spinner on the submit button, matching the side-modal forms.

Candidates for ModalForm

• ResizeInstanceModal — app/pages/project/instances/InstancePage.tsx:274. Has an inline <p className="text-error"> at :389 that the shell replaces.
• SiloSubnetPoolsTab Link pool — app/pages/system/silos/SiloSubnetPoolsTab.tsx:297
• SiloIpPoolsTab Link pool — app/pages/system/silos/SiloIpPoolsTab.tsx:311
• IpPoolPage Link silo — app/pages/system/networking/IpPoolPage.tsx:493
• SubnetPoolPage Link silo — app/pages/system/networking/SubnetPoolPage.tsx:475
• AntiAffinityCard Add member — app/pages/project/instances/AntiAffinityCard.tsx:241
• anti-affinity-group-member-add Add instance — app/forms/anti-affinity-group-member-add.tsx:67

ModalForm hardcodes <Message title="Error"> inline — simplest to drop the custom titles, matching #3197. Or we could add the ability to pass a custom error title.

Doesn't fit — not form submissions

These have no inputs and would need a throwaway useForm just to satisfy ModalForm's signature:

• SiloScimTab Create token — app/pages/system/silos/SiloScimTab.tsx:220. Informational body; Create fires the mutation and onSuccess hands the token to a second modal.
• ImagesPage Promote image — app/pages/project/images/ImagesPage.tsx:189. "Are you sure you want to promote X?"

Leave as-is or refactor toward confirmAction.