fix(popover): prevent right/bottom overflow and add screenGap parameter

[Copilot]

Popover currently constrains only left-side overflow (Math.max(left, 0)), allowing content to overflow right and bottom edges of the viewport.

Changes

• Add screenGap property (default: 0) - configurable spacing between popover and screen edges when constrained
• Fix position calculation - constrain all four sides:

  // Before: only left side constrained
  const style = `top:${Math.max(top, 0)}px;left:${Math.max(left, 0)}px;`;
  
  // After: all sides constrained with edge case handling
  const maxLeft = Math.max(screenGap, windowWidth - contentWidth - screenGap);
  const maxTop = Math.max(screenGap, windowHeight - contentHeight - screenGap);
  const constrainedLeft = Math.min(Math.max(left, screenGap), maxLeft);
  const constrainedTop = Math.min(Math.max(top, screenGap), maxTop);

• Handle viewport overflow - when popover exceeds viewport dimensions, Math.max(screenGap, ...) ensures positioning never goes negative
• Apply to both platforms - changes implemented in both packages/components/popover/ (miniprogram) and packages/uniapp-components/popover/ (uniapp)

Usage

<!-- Default behavior (backward compatible) -->
<t-popover placement="top-right" visible="{{visible}}">
  <view slot="content">Content won't overflow screen edges</view>
</t-popover>

<!-- Custom spacing from edges -->
<t-popover screen-gap="16" placement="top-right" visible="{{visible}}">
  <view slot="content">Content keeps 16px from edges</view>
</t-popover>

Backward compatible - default screenGap: 0 maintains existing behavior.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[t-popover] 气泡支持了不超出左侧屏幕，但没有支持不超出右侧屏幕</issue_title>
<issue_description>### 这个功能解决了什么问题

当前气泡在超出左侧屏幕时默认紧靠左侧屏幕，但是超出右侧屏幕时没有紧靠右侧

你建议的方案是什么

1. 气泡超出右侧时紧靠右侧屏幕。
2. 提供一个参数来调整紧靠屏幕时与屏幕的间距</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [t-popover] 气泡支持了不超出左侧屏幕，但没有支持不超出右侧屏幕 #4197

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