fix: style preview real-time — CSS vars in theme blocks.css + clearStyleField deletes key

Root causes:
1. Font/color CSS consumption rules were added to platform blocks.css
   but editor preview uses the THEME's blocks.css (which lacked them)
2. Reset buttons sent empty string which failed validation → 400 →
   preview died silently

Fix: Added --sf-title-size/--sf-text-size/--sf-accent-c/--sf-btn-c
consumption rules to themes/trade-pro/blocks.css; reset now uses
clearStyleField() which deletes the key instead of setting ''.

Co-Authored-By: Claude Code <noreply@anthropic.com>

themes/trade-pro/blocks.css

@@ -445,3 +445,11 @@
     min-height: min(64vh, 480px);
   }
 }
+
+/* ── 区块级字号/颜色自定义（编辑器实时预览消费）── */
+.sf-section h1,.sf-section h2{font-size:var(--sf-title-size,clamp(38px,5vw,76px))!important}
+.sf-section h3{font-size:calc(var(--sf-title-size,28px) * 0.6)!important}
+.sf-section p,.sf-section li{font-size:var(--sf-text-size,16px)!important}
+.sf-section .sf-kicker{color:var(--sf-accent-c,#3569d3)!important}
+.sf-section .sf-link,.sf-section .sf-section-head>a,.sf-section .sf-item-copy small{color:var(--sf-accent-c,#3569d3)!important}
+.sf-section .sf-button{background:var(--sf-btn-c,var(--accent,#315ee2))!important}