Show file-upload validation error in modal instead of inline 'Invalid Input'

[akash-m-2001]

Summary

When a user clicks Calculate on the new-case page without choosing an energy-use file, conform's client-side validation fails and printed a raw `Invalid input` next to the file picker — meaningless to a homeowner.

This replaces that inline text with the existing `ErrorModal` and homeowner-readable copy telling them to download a CSV from their utility and upload it. The modal clears automatically once a valid file is selected (conform revalidates on input).

• Client-side only, because conform blocks the submit before it reaches the action.
• Edit mode is unaffected (`!isEditMode` guard).
• The server-side modal for bad CSV content (wrong format, malformed dates) is unchanged.

Part of #699.

Test plan

• `/cases/new`, fill the form, don't choose a file, click Calculate → friendly modal, no red inline text
• Choose a valid CSV → modal closes, calculation proceeds
• Upload a malformed CSV → server-side modal still works
• `npm run typecheck` passes
• `SingleCaseForm.test.tsx`: same pass/fail as main (pre-existing jsdom failures unrelated)

[EkaterinaKladova]

Front end works, error shows up the first time Calculate button is clicked without a file uploaded. Looks good!