Per-field config: placeholder + max-length counter with hard-block

What to build

Add per-field configuration and length handling on top of the Lexical editor.

A config screen (rendered via the SDK's manual-field-extension config parameters) exposes:

• Placeholder text — shown when the field is empty
• Optional max character count

Length behavior in the editor:

• Live "X/Y" counter that counts visible text only (bold markup does not count toward the limit)
• Hard-block input once the limit is reached — further typing and paste beyond the max are rejected

Note: a field-editor plugin cannot hook Dato's save-time validation, so the limit is enforced at the input level, not by blocking record save.

Type: AFK

Acceptance criteria

• Config screen lets an admin set a placeholder and an optional max length per field
• Placeholder renders in the editor when the value is empty
• Live counter shows visible-text length as "X/Y" (bold markup excluded)
• Typing is hard-blocked at the max; paste is truncated/rejected so the value never exceeds the limit
• With no max configured, the counter/limit behavior is disabled gracefully

Blocked by

• #3

Start by resolving the dependency on issue #3, then inspect the SDK's manual-field-extension config parameters and the Lexical editor integration. Done means the config screen, placeholder, visible-text counter, input and paste hard limit, and disabled no-max behavior all satisfy the listed acceptance criteria.