Fix empty-looking user search and docassemble 1.9 picker crash

[nonprofittechy]

On servers with more than 200 users, Manage users switches to an AJAX picker that previously appeared empty until an administrator entered four characters of an email prefix. Apps-dev confirmed that two-character searches produced no results. Start searches after two characters and show visible instructions beneath the field; also allow matching names and text within email addresses.

Apps-dev's logs revealed a separate docassemble 1.9 rendering crash after submitting without a user selection: the AJAX field's integer default reached a string-only renderer. Keep picker values as text and derive the numeric user ID separately, invalidating it when the selection changes. Preserve bounded results and privileged-account filtering.

Validation:

• 47 focused user-management, session, and compatibility tests passed.
• Local browser check verified visible guidance, two-character AJAX selection, numeric ID conversion, and empty-selection validation.
• Apps-dev's exact 1.9.8.post5 renderer reproduced the original integer-default crash and rendered the replacement text values successfully.
• YAML checker: no errors; four existing warnings. Black and git diff checks passed.

Not deployed to apps-dev.