✨ Display progress bar or loading indicator in Open Quickly

### Description

After opening a large monorepo, I thought the Open Quickly feature was broken because it took several seconds to perform a search, and there was no indication that a search was being performed in the background.

### Alternatives Considered

_No response_

### Additional Context

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### Screenshots

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Start by locating the Open Quickly search flow in the CodeEdit macOS app and tracing how background searches are initiated. Add a visible progress or loading indicator while a search is running, and verify that it appears during slow searches and stops when results are ready.

Written by the indexing model from the issue text.