Extends search-command beyond search #687

Right now search-command is search-only. Idea: extend it to support actions too, basically turn it into a command palette.

Would let users do things like:

• toggle dark/light theme
• enable/disable flags or warnings
• apply preset filters ("only flagged", "only high severity"...)
• clear cache / clear packages in the SearchNav

Key constraint: actions dispatched from the palette must go through the same mechanism as the settings panel, no duplicated state logic.

Probably worth renaming the component too (command-palette? command-bar?) since search-command won't cover it anymore.