[16.0][FIX] disbursement_finance_kmitl: unblock navigation at bills_posted when payment subject is empty

[coreichamp]

Summary

• payment_subject_id (เรื่องที่จ่าย) was required in the view exactly at bills_posted, the one state every request arrives at with the field empty (the accounting bridge writes that state; choosing the subject is the auditor's own first job).
• Every type="object" button (header buttons and smart buttons alike) saves and validates the whole form regardless of whether it's dirty, so the auditor's smart buttons (ใบตั้งหนี้ / payments / journal entries / sarabun) were blocked on an untouched form — locking the auditor out of the very documents they need to decide the subject.
• The requirement is already enforced server-side by _check_payment_classification, the only transition that consumes it (action_audit, action_audit_batch, _create_payments), with its own translated UserError and existing test coverage (test_audit_needs_a_subject).
• Dropped the view-level required (kept readonly), and replaced the lost asterisk with a non-blocking warning banner on the Payment Execution page reusing the exact guard message, so the translation is reused (added the view #: reference to the existing th.po entry rather than creating a duplicate).

No model, security, or test changes — the guard, its message, and its test already existed.

Test plan

• Restart with -u disbursement_finance_kmitl so the view and translation reload.
• Take a DR to bills_posted, open it from the audit queue with เรื่องที่จ่าย empty, on an untouched form: each smart button opens instead of raising an "Invalid fields" toast; breadcrumb back to list works.
• Dirty the form (click into a payment line) and repeat, plus a pager arrow.
• Warning banner is visible on Payment Execution page throughout.
• Press ตรวจสอบการเบิกจ่าย with no subject → UserError, state stays bills_posted.
• Select a subject → banner disappears, rows derive their หัวจ่าย, audit succeeds.
• Batch ตรวจสอบ from the audit queue on a subject-less request → same UserError, nothing advances.
• oca_run_tests limited to disbursement_finance_kmitl stays green.