fix(form): validate conflicting field options

[oxura]

Closes #294.

Form fields currently accept contradictory custom options and only complain later while cleaning a submitted value. That makes an invalid field definition look healthy until a request happens to exercise it.

This moves the invariant to field construction. Text length ranges, numeric ranges, and date/time ranges now reject min > max from FormField::with_options, while equal bounds and unconstrained fields keep working as before. The shared field macro calls a small internal validation trait, so each field family owns its option invariant without duplicating construction logic.

I also replaced the old delayed email-validation test and added construction-time coverage for every constrained field family.

Checked with:

• cargo test -p cot --all-features form::fields (113 passed)
• cargo clippy -p cot --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• git diff --check