feat(theme): support overflow-based frozen column shadows

[kitee0325]

🤔 This is a ...

• New feature
• TypeScript definition update
• Test Case
• Site / documentation update
• Demo update

🔗 Related issue link

No existing issue. Related example: https://visactor.io/vtable/demo/basic-functionality/frozen-col

💡 Background and solution

Frozen-column shadows can currently stay visible (always) or follow the scrolling UI (scrolling). Neither mode indicates whether body content is hidden beyond a frozen boundary. A left frozen column can retain a shadow at scrollLeft = 0, while a scrolling shadow disappears after the scrollbar's hide timer even though content is still covered.

Add the opt-in theme.frozenColumnLine.shadow.visible: 'overflow' mode:

• No frozen columns or no horizontal body overflow: no shadows.
• Left frozen boundary: show after leaving the horizontal start.
• Right frozen boundary: show while body content remains to the right; hide at the end.
• Keep visibility after scrolling stops; update immediately when returning to an edge.

theme: VTable.themes.DEFAULT.extends({
  frozenColumnLine: {
    shadow: {
      width: 8,
      startColor: 'rgba(0, 0, 0, 0.12)',
      endColor: 'rgba(0, 0, 0, 0)',
      visible: 'overflow'
    }
  }
})

Visibility follows the existing body scroll range, including separately scrollable frozen viewports and fractional-width tolerance. Updates run after accepted scroll-position changes and layout updates. Existing gradient rectangles are reused, with no new event subscriptions or timers. The built-in default theme and the behavior of always / scrolling are unchanged.

English and Chinese option docs and frozen-column demos are updated. The documented default is corrected from scrolling to the existing always.

Validation

• 5 Jest Electron suites, 30 tests passed, including 11 new shadow cases.
• Covered programmatic/scrollbar movement, no-frozen/no-overflow states, dynamic freezing, resize, theme/updateOption, cancelled scrolling, fractional widths, independently scrollable frozen columns, and PivotTable boundaries.
• TypeScript compile, ESLint (0 errors; existing warnings), Prettier, Git commit hooks, and rush change --verify --target-branch origin/develop --no-fetch passed.
• Chromium rendering checked at the start, middle and end, with and without frozen columns.

📝 Changelog

Language	Changelog
🇺🇸 English	Add opt-in overflow-based frozen column shadow visibility; existing modes and default theme remain unchanged.
🇨🇳 Chinese	新增按内容溢出状态显示冻结列阴影的可选模式，保留现有模式和默认主题行为。

☑️ Self-Check before Merge

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• TypeScript definition is updated/provided or not needed
• Changelog is provided or not needed

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🚀 Summary

copilot:summary

🔍 Walkthrough

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Rendering comparison

[kitee0325]

@fangsmile 大佬看下是否满足合入条件