Fix WebUI Playbook editor font scale

[edwin-zvs]

Summary

• reduce the WebUI Playbook editor body font from 14px to 13px so its computed size matches the surrounding Playbook/WebUI typography
• preserve the editor monospace stack, 1.6 line height, wrapping, and responsive behavior
• add a computed-style regression asserting the editor/surrounding size relationship and monospace readability

Root cause

The Playbook contenteditable had a standalone 14px font-size declaration while its surrounding Playbook toolbar and other dense WebUI surfaces compute to 13px. The monospace face made that one-pixel mismatch especially noticeable.

Tests

• cargo test -p construct-e2e --test playbook_view -- --nocapture (8 passed)
• cargo test -p construct-e2e --test web_smoke -- --nocapture (2 passed; concurrent Chromium singleton skip rerun separately)
• cargo test -p construct-e2e --test web_smoke web_operator_editor_round_trips_slack_personal_response_mode_overrides -- --nocapture --exact (1 passed)
• cargo test -p construct-e2e --test playbook_view web_playbook_view_full_parity -- --nocapture --exact (1 passed, final diff)
• cargo build

[edwin-zvs]

Deterministic isolated WebUI capture (800×600; test daemon, no live user state).

Before (14px editor):

After (13px editor):