🎨 Palette: Add tooltips to icon-only PopupMenuButtons

[JustLookAtNow]

💡 What: Added tooltip: '选项' to PopupMenuButton widgets in the server settings page and aggregate search settings page. Cleaned up duplicate tooltip attributes in download tasks page.
🎯 Why: Icon-only menus (like PopupMenuButton) lack semantic meaning for screen readers and don't show hover feedback on desktop platforms unless a tooltip string is explicitly provided. This improves overall keyboard accessibility and UI discoverability.
📸 Before/After: Visually unchanged on mobile unless long-pressed; on desktop/web, hovering over the context menu icons now displays "选项" (Options) instead of nothing.
♿ Accessibility: Provides necessary ARIA-equivalent labels to screen readers for critical settings actions.

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PR created automatically by Jules for task 1205530211742918021 started by @JustLookAtNow

[google-labs-jules]

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