fix: 浮动工具栏精确定位 — 紧贴选中文字 4px，顶部不足时自动显示在下方

src/core.ts

@@ -737,12 +737,17 @@ export class MarkdownEditor {
       const paddingTop = parseInt(getComputedStyle(ta).paddingTop, 10) || 16;
       const scrollOffset = ta.scrollTop;
 
-      // Place toolbar just above the selected line (4px gap)
+      // Line top relative to textarea content area
+      const lineTop = rect.top + paddingTop + currentLine * lineHeight - scrollOffset;
       const toolbarHeight = 36;
-      let top = rect.top + paddingTop + currentLine * lineHeight - scrollOffset - toolbarHeight - 4;
-      // Clamp: don't go above textarea
-      if (top < rect.top) top = rect.top + 4;
-      if (top + toolbarHeight > rect.bottom) top = rect.bottom - toolbarHeight - 4;
+      const gap = 4;
+
+      // Try above first
+      let top = lineTop - toolbarHeight - gap;
+      // If not enough room above, place below the selection
+      if (top < rect.top + gap) {
+        top = lineTop + lineHeight + gap;
+      }
 
       // Center horizontally near the middle of selection
       let left = rect.left + (ta.selectionDirection !== 'backward' ? (ta.selectionEnd - ta.selectionStart) / 2 : 0) * 8 + 10;