fix(custom-fields): tri-state bool input, case-insensitive bool default coerce, multi_select default uses | separator

[jamofer]

Summary

Three custom-field UX bugs from the same surface, fixed together so the form/admin pair stays consistent.

• Bool input was indistinguishable from default. FieldControl rendered a <input type=checkbox> for bool fields, so value=false and "user hasn't touched it yet" looked identical and there was no way to express "use the field's default". Replaced with a tri-state <Select> that always exposes three options:
  • Yes → emits true
  • No → emits false
  • Not set / Default (Yes|No) → emits undefined so the backend's default_value wins. The label folds in the declared default when one is set, so admins immediately see what "Not set" actually means in this project.
• Bool coerceDefault was case-sensitive. The dialog only accepted literal "true" / "1"; "True" / "TRUE" / "yes" silently saved false. The new helper lowercases + trims and accepts true|1|yes|y|on|sí|si|s as true, false|0|no|n|off as false, anything else as false.
• multi_select default split on commas. Option values are allowed to contain commas (e.g. "web, mobile"), so the old raw.split(",") produced bogus values. Switched the default-value separator to | (already the convention used in the options input), updated the placeholder + hint per type, and the helper now String(raw).split("|").trim().filter(Boolean).

Refactor:

• Extracted coerceDefault from CustomFieldsPanel.jsx into frontend/src/lib/customFieldDefaults.js so the rules are unit-testable in isolation. CustomFieldsPanel.jsx just imports it.
• New per-type default placeholder + hint helper avoids the generic message lying about acceptable input.

Tests:

• customFieldDefaults.test.js covers bool true/false case variants, multi_select pipe split with commas inside values, whitespace trimming, plus the number/text passthrough.
• CustomFieldInput.test.jsx updated: dropped the obsolete checkbox test, added three new cases — pick Yes emits true, pick No emits false (distinct from unset), and the unset option label surfaces the declared default ("Default (No)").