保持弹窗打开时的位置稳定

[1254455745]

这个 PR 调整菜单栏弹窗的锚点逻辑，避免菜单栏内容宽度变化时弹窗左右跳动。

改动内容：

• 弹窗打开时锁定本次状态栏项目宽度。
• 菜单栏内容切换期间，弹窗位置保持稳定。
• 弹窗关闭后恢复正常宽度更新逻辑。

验证：

• 单独分支构建通过。

[pullfrog]

✅ No new issues found.

Reviewed changes — locks NSStatusItem length while the popover is open so the anchor position stays stable when ticker content width changes. Also tightens NotificationCenter observer lifecycle in PopoverController.

• Lock popover anchor width — StatusItemController.lockedPopoverLength overrides tickerView.totalWidth in all sizing paths while non-nil, set before showing the popover and cleared on close.
• Popover close callback — PopoverController.onClose wired to unlockPopoverLength(), ensuring the width lock is released regardless of how the popover closes (.transient behavior, programmatic close, or outside-click).
• Observer lifecycle fix — didCloseObserver correctly stores the block-based observer token for targeted removal in deinit; previous removeObserver(self) was a no-op for block-based observers.
• MainActor safety — handlePopoverClosed wrapped in Task { @MainActor } in the notification callback, guarding against non-main-thread delivery.

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