🎨 Palette: Add Command Palette trigger button to sidebar

[TheDragonSoft]

Adds a search trigger button with Ctrl K shortcut indicator to the Sidebar, improving discoverability and keyboard accessibility for the Command Palette feature. Includes aria-label and aria-keyshortcuts attributes.

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PR created automatically by Jules for task 6946636583079849802 started by @TheDragonSoft

[google-labs-jules]

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