[16.0][IMP] multiple: replace attrs readonly with can_edit computed field

[madara1150]

Summary

• Refactor `attrs="{'readonly': [('state', ...)]}"\ patterns across 7 custom modules into a `can_edit` computed Boolean field on each model
• Centralizes readonly logic in Python (model layer) instead of duplicating state conditions across every XML field
• Future state changes only require updating a single `_compute_can_edit` method per model

Modules changed

Module	Model	Fields refactored
`advance_payment`	`advance.payment`	4
`agx_sarabun`	`sarabun.document`	5
`budget`	`budget.transfer`	6
`budget_appropriation`	`budget.appropriation`	2
`hr_recruitment_kmitl`	`hr.onboarding`	10
`procurement_plan`	`procurement.plan`	6 (reuse existing `can_edit`)
`purchase_request_approval_disbursement`	`purchase.request.approval`	1
Total		34 fields

Pattern

Before (XML):
```xml

```

After (Python model):
```python
can_edit = fields.Boolean(compute="_compute_can_edit")

@api.depends("state")
def _compute_can_edit(self):
for rec in self:
rec.can_edit = rec.state == "draft"
```

After (XML):
```xml


```

Out of scope

• `kris_project` / `kris_project_budget` — handled in a separate PR
• `disbursement` — complex OR conditions combining state with other fields
• `agx_approval`, `purchase_request_budget` — already use `is_budget_editable` computed field
• Standard Odoo model inheritors (`accounting_kmitl`, `finance_kmitl`, `purchase_budget`, etc.)
• Two fields in `advance_payment` with unique single-use conditions (`return_line_ids`, `attachment_ids`)

Test plan

• Open each affected form view and verify fields are editable in `draft` state
• Transition record out of `draft` and verify fields become readonly
• Confirm `can_edit` field is not visible in the UI
• `procurement_plan`: verify both the existing `can_edit` fields and the 6 newly migrated fields behave consistently